diff --git a/tests/VObject/AttachIssueTest.php b/tests/VObject/AttachIssueTest.php
index 74c93cd0..6f8981a3 100644
--- a/tests/VObject/AttachIssueTest.php
+++ b/tests/VObject/AttachIssueTest.php
@@ -17,6 +17,6 @@ public function testRead(): void
 
 ICS;
         $obj = Reader::read($event);
-        $this->assertEquals($event, $obj->serialize());
+        self::assertEquals($event, $obj->serialize());
     }
 }
diff --git a/tests/VObject/BirthdayCalendarGeneratorTest.php b/tests/VObject/BirthdayCalendarGeneratorTest.php
index 3ebc6d98..ac69d1f0 100644
--- a/tests/VObject/BirthdayCalendarGeneratorTest.php
+++ b/tests/VObject/BirthdayCalendarGeneratorTest.php
@@ -39,7 +39,7 @@ public function testVcardStringWithValidBirthday(): void
         $generator->setObjects($input);
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
@@ -97,7 +97,7 @@ public function testArrayOfVcardStringsWithValidBirthdays(): void
         $generator->setObjects($input);
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
@@ -156,7 +156,7 @@ public function testArrayOfVcardStringsWithValidBirthdaysViaConstructor(): void
         $generator->setObjects($input);
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
@@ -195,7 +195,7 @@ public function testVcardObjectWithValidBirthday(): void
         $generator->setObjects($input);
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
@@ -257,7 +257,7 @@ public function testArrayOfVcardObjectsWithValidBirthdays(): void
         $generator->setObjects($input);
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
@@ -294,7 +294,7 @@ public function testVcardStringWithValidBirthdayWithXAppleOmitYear(): void
         $generator->setObjects($input);
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
@@ -331,7 +331,7 @@ public function testVcardStringWithValidBirthdayWithoutYear(): void
         $generator->setObjects($input);
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
@@ -359,7 +359,7 @@ public function testVcardStringWithInvalidBirthday(): void
         $generator->setObjects($input);
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
@@ -386,7 +386,7 @@ public function testVcardStringWithNoBirthday(): void
         $generator->setObjects($input);
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
@@ -424,7 +424,7 @@ public function testVcardStringWithValidBirthdayLocalized(): void
         $generator->setFormat('%1$s\'s Geburtstag');
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
@@ -452,7 +452,7 @@ public function testVcardStringWithEmptyBirthdayProperty(): void
         $generator->setObjects($input);
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
@@ -534,7 +534,7 @@ public function testBrokenVcardWithoutFN(): void
         $generator->setObjects($input);
         $output = $generator->getResult();
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $output
         );
diff --git a/tests/VObject/CliTest.php b/tests/VObject/CliTest.php
index 083f3601..c57d2522 100644
--- a/tests/VObject/CliTest.php
+++ b/tests/VObject/CliTest.php
@@ -28,89 +28,89 @@ public function setUp(): void
 
     public function testInvalidArg(): void
     {
-        $this->assertEquals(
+        self::assertEquals(
             1,
             $this->cli->main(['vobject', '--hi'])
         );
         rewind($this->cli->stderr);
-        $this->assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
+        self::assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
     }
 
     public function testQuiet(): void
     {
-        $this->assertEquals(
+        self::assertEquals(
             1,
             $this->cli->main(['vobject', '-q'])
         );
-        $this->assertTrue($this->cli->quiet);
+        self::assertTrue($this->cli->quiet);
 
         rewind($this->cli->stderr);
-        $this->assertEquals(0, strlen(stream_get_contents($this->cli->stderr)));
+        self::assertEquals(0, strlen(stream_get_contents($this->cli->stderr)));
     }
 
     public function testHelp(): void
     {
-        $this->assertEquals(
+        self::assertEquals(
             0,
             $this->cli->main(['vobject', '-h'])
         );
         rewind($this->cli->stderr);
-        $this->assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
+        self::assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
     }
 
     public function testFormat(): void
     {
-        $this->assertEquals(
+        self::assertEquals(
             1,
             $this->cli->main(['vobject', '--format=jcard'])
         );
 
         rewind($this->cli->stderr);
-        $this->assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
+        self::assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
 
-        $this->assertEquals('jcard', $this->cli->format);
+        self::assertEquals('jcard', $this->cli->format);
     }
 
     public function testFormatInvalid(): void
     {
-        $this->assertEquals(
+        self::assertEquals(
             1,
             $this->cli->main(['vobject', '--format=foo'])
         );
 
         rewind($this->cli->stderr);
-        $this->assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
+        self::assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
 
-        $this->assertNull($this->cli->format);
+        self::assertNull($this->cli->format);
     }
 
     public function testInputFormatInvalid(): void
     {
-        $this->assertEquals(
+        self::assertEquals(
             1,
             $this->cli->main(['vobject', '--inputformat=foo'])
         );
 
         rewind($this->cli->stderr);
-        $this->assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
+        self::assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
 
-        $this->assertNull($this->cli->format);
+        self::assertNull($this->cli->format);
     }
 
     public function testNoInputFile(): void
     {
-        $this->assertEquals(
+        self::assertEquals(
             1,
             $this->cli->main(['vobject', 'color'])
         );
 
         rewind($this->cli->stderr);
-        $this->assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
+        self::assertTrue(strlen(stream_get_contents($this->cli->stderr)) > 100);
     }
 
     public function testTooManyArgs(): void
     {
-        $this->assertEquals(
+        self::assertEquals(
             1,
             $this->cli->main(['vobject', 'color', 'a', 'b', 'c'])
         );
@@ -118,7 +118,7 @@ public function testTooManyArgs(): void
 
     public function testUnknownCommand(): void
     {
-        $this->assertEquals(
+        self::assertEquals(
             1,
             $this->cli->main(['vobject', 'foo', '-'])
         );
@@ -138,14 +138,14 @@ public function testConvertJson(): void
         rewind($inputStream);
         $this->cli->stdin = $inputStream;
 
-        $this->assertEquals(
+        self::assertEquals(
             0,
             $this->cli->main(['vobject', 'convert', '--format=json', '-'])
         );
 
         rewind($this->cli->stdout);
         $version = Version::VERSION;
-        $this->assertEquals(
+        self::assertEquals(
             '["vcard",[["version",{},"text","4.0"],["prodid",{},"text","-\/\/Sabre\/\/Sabre VObject '.$version.'\/\/EN"],["fn",{},"text","Cowboy Henk"]]]',
             stream_get_contents($this->cli->stdout)
         );
@@ -169,7 +169,7 @@ public function testConvertJCardPretty(): void
         rewind($inputStream);
         $this->cli->stdin = $inputStream;
 
-        $this->assertEquals(
+        self::assertEquals(
             0,
             $this->cli->main(['vobject', 'convert', '--format=jcard', '--pretty', '-'])
         );
@@ -186,7 +186,7 @@ public function testConvertJCardPretty(): void
             "versi
 JCARD;
 
-        $this->assertStringStartsWith(
+        self::assertStringStartsWith(
             $expected,
             stream_get_contents($this->cli->stdout)
         );
@@ -206,7 +206,7 @@ public function testConvertJCalFail(): void
         rewind($inputStream);
         $this->cli->stdin = $inputStream;
 
-        $this->assertEquals(
+        self::assertEquals(
             2,
             $this->cli->main(['vobject', 'convert', '--format=jcal', '--inputformat=mimedir', '-'])
         );
@@ -251,7 +251,7 @@ public function testConvertMimeDir(): void
         rewind($inputStream);
         $this->cli->stdin = $inputStream;
 
-        $this->assertEquals(
+        self::assertEquals(
             0,
             $this->cli->main(['vobject', 'convert', '--format=mimedir', '--inputformat=json', '--pretty', '-'])
         );
@@ -266,7 +266,7 @@ public function testConvertMimeDir(): void
 
 VCF;
 
-        $this->assertEquals(
+        self::assertEquals(
             strtr($expected, ["\n" => "\r\n"]),
             stream_get_contents($this->cli->stdout)
         );
@@ -276,26 +276,26 @@ public function testConvertDefaultFormats(): void
     {
         $outputFile = $this->sabreTempDir.'bar.json';
 
-        $this->assertEquals(
+        self::assertEquals(
             2,
             $this->cli->main(['vobject', 'convert', 'foo.json', $outputFile])
         );
 
-        $this->assertEquals('json', $this->cli->inputFormat);
-        $this->assertEquals('json', $this->cli->format);
+        self::assertEquals('json', $this->cli->inputFormat);
+        self::assertEquals('json', $this->cli->format);
     }
 
     public function testConvertDefaultFormats2(): void
     {
         $outputFile = $this->sabreTempDir.'bar.ics';
 
-        $this->assertEquals(
+        self::assertEquals(
             2,
             $this->cli->main(['vobject', 'convert', 'foo.ics', $outputFile])
         );
 
-        $this->assertEquals('mimedir', $this->cli->inputFormat);
-        $this->assertEquals('mimedir', $this->cli->format);
+        self::assertEquals('mimedir', $this->cli->inputFormat);
+        self::assertEquals('mimedir', $this->cli->format);
     }
 
     public function testVCard3040(): void
@@ -314,7 +314,7 @@ public function testVCard3040(): void
         rewind($inputStream);
         $this->cli->stdin = $inputStream;
 
-        $this->assertEquals(
+        self::assertEquals(
             0,
             $this->cli->main(['vobject', 'convert', '--format=vcard40', '--pretty', '-'])
         );
@@ -331,7 +331,7 @@ public function testVCard3040(): void
 
 VCF;
 
-        $this->assertEquals(
+        self::assertEquals(
             strtr($expected, ["\n" => "\r\n"]),
             stream_get_contents($this->cli->stdout)
         );
@@ -353,7 +353,7 @@ public function testVCard4030(): void
         rewind($inputStream);
         $this->cli->stdin = $inputStream;
 
-        $this->assertEquals(
+        self::assertEquals(
             0,
             $this->cli->main(['vobject', 'convert', '--format=vcard30', '--pretty', '-'])
         );
@@ -370,7 +370,7 @@ public function testVCard4030(): void
 
 VCF;
 
-        $this->assertEquals(
+        self::assertEquals(
             strtr($expected, ["\n" => "\r\n"]),
             stream_get_contents($this->cli->stdout)
         );
@@ -392,7 +392,7 @@ public function testVCard4021(): void
         rewind($inputStream);
         $this->cli->stdin = $inputStream;
 
-        $this->assertEquals(
+        self::assertEquals(
             2,
             $this->cli->main(['vobject', 'convert', '--format=vcard21', '--pretty', '-'])
         );
@@ -416,7 +416,7 @@ public function testValidate(): void
         $this->cli->stdin = $inputStream;
         $result = $this->cli->main(['vobject', 'validate', '-']);
 
-        $this->assertEquals(
+        self::assertEquals(
             0,
             $result
         );
@@ -436,7 +436,7 @@ public function testValidateFail(): void
         rewind($inputStream);
         $this->cli->stdin = $inputStream;
         // vCard 2.0 is not supported yet, so this returns a failure.
-        $this->assertEquals(
+        self::assertEquals(
             2,
             $this->cli->main(['vobject', 'validate', '-'])
         );
@@ -456,7 +456,7 @@ public function testValidateFail2(): void
         rewind($inputStream);
         $this->cli->stdin = $inputStream;
 
-        $this->assertEquals(
+        self::assertEquals(
             2,
             $this->cli->main(['vobject', 'validate', '-'])
         );
@@ -476,7 +476,7 @@ public function testRepair(): void
         rewind($inputStream);
         $this->cli->stdin = $inputStream;
 
-        $this->assertEquals(
+        self::assertEquals(
             2,
             $this->cli->main(['vobject', 'repair', '-'])
         );
@@ -484,7 +484,7 @@ public function testRepair(): void
         rewind($this->cli->stdout);
         $regularExpression = "/^BEGIN:VCARD\r\nVERSION:2.1\r\nUID:[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}\r\nEND:VCARD\r\n$/";
         $data = stream_get_contents($this->cli->stdout);
-        $this->assertMatchesRegularExpression($regularExpression, $data);
+        self::assertMatchesRegularExpression($regularExpression, $data);
     }
 
     public function testRepairNothing(): void
@@ -512,7 +512,7 @@ public function testRepairNothing(): void
         rewind($this->cli->stderr);
         $error = stream_get_contents($this->cli->stderr);
 
-        $this->assertEquals(
+        self::assertEquals(
             0,
             $result,
             "This should have been error free. stderr output:\n".$error
@@ -558,7 +558,7 @@ public function testColorCalendar(): void
         rewind($this->cli->stderr);
         $error = stream_get_contents($this->cli->stderr);
 
-        $this->assertEquals(
+        self::assertEquals(
             0,
             $result,
             "This should have been error free. stderr output:\n".$error
@@ -599,7 +599,7 @@ public function testColorVCard(): void
         rewind($this->cli->stderr);
         $error = stream_get_contents($this->cli->stderr);
 
-        $this->assertEquals(
+        self::assertEquals(
             0,
             $result,
             "This should have been error free. stderr output:\n".$error
diff --git a/tests/VObject/Component/AvailableTest.php b/tests/VObject/Component/AvailableTest.php
index ce3f6ed6..98202ffc 100644
--- a/tests/VObject/Component/AvailableTest.php
+++ b/tests/VObject/Component/AvailableTest.php
@@ -20,7 +20,7 @@ public function testAvailableComponent(): void
 END:VCALENDAR
 VCAL;
         $document = Reader::read($vcal);
-        $this->assertInstanceOf(Available::class, $document->AVAILABLE);
+        self::assertInstanceOf(Available::class, $document->AVAILABLE);
     }
 
     public function testGetEffectiveStartEnd(): void
@@ -36,7 +36,7 @@ public function testGetEffectiveStartEnd(): void
 
         $document = Reader::read($vcal);
         $tz = new \DateTimeZone('UTC');
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2015-07-17 16:22:00', $tz),
                 new \DateTimeImmutable('2015-07-17 17:22:00', $tz),
@@ -58,7 +58,7 @@ public function testGetEffectiveStartEndDuration(): void
 
         $document = Reader::read($vcal);
         $tz = new \DateTimeZone('UTC');
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2015-07-17 16:22:00', $tz),
                 new \DateTimeImmutable('2015-07-17 17:22:00', $tz),
diff --git a/tests/VObject/Component/VAlarmTest.php b/tests/VObject/Component/VAlarmTest.php
index d59fa6cc..4694d636 100644
--- a/tests/VObject/Component/VAlarmTest.php
+++ b/tests/VObject/Component/VAlarmTest.php
@@ -13,7 +13,7 @@ class VAlarmTest extends TestCase
      */
     public function testInTimeRange(VAlarm $valarm, \DateTime $start, \DateTime $end, bool $outcome): void
     {
-        $this->assertEquals($outcome, $valarm->isInTimeRange($start, $end));
+        self::assertEquals($outcome, $valarm->isInTimeRange($start, $end));
     }
 
     public function timeRangeTestData(): array
@@ -165,6 +165,6 @@ public function testInTimeRangeBuggy(): void
 
         $vobj = Reader::read($input);
 
-        $this->assertTrue($vobj->VTODO->VALARM->isInTimeRange(new \DateTime('2012-10-01 00:00:00'), new \DateTime('2012-11-01 00:00:00')));
+        self::assertTrue($vobj->VTODO->VALARM->isInTimeRange(new \DateTime('2012-10-01 00:00:00'), new \DateTime('2012-11-01 00:00:00')));
     }
 }
diff --git a/tests/VObject/Component/VAvailabilityTest.php b/tests/VObject/Component/VAvailabilityTest.php
index 30ec3d56..263eeebd 100644
--- a/tests/VObject/Component/VAvailabilityTest.php
+++ b/tests/VObject/Component/VAvailabilityTest.php
@@ -22,7 +22,7 @@ public function testVAvailabilityComponent(): void
 VCAL;
         $document = Reader::read($vcal);
 
-        $this->assertInstanceOf(VAvailability::class, $document->VAVAILABILITY);
+        self::assertInstanceOf(VAvailability::class, $document->VAVAILABILITY);
     }
 
     public function testGetEffectiveStartEnd(): void
@@ -38,7 +38,7 @@ public function testGetEffectiveStartEnd(): void
 
         $document = Reader::read($vcal);
         $tz = new \DateTimeZone('UTC');
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2015-07-17 16:22:00', $tz),
                 new \DateTimeImmutable('2015-07-17 17:22:00', $tz),
@@ -60,7 +60,7 @@ public function testGetEffectiveStartDuration(): void
 
         $document = Reader::read($vcal);
         $tz = new \DateTimeZone('UTC');
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2015-07-17 16:22:00', $tz),
                 new \DateTimeImmutable('2015-07-17 17:22:00', $tz),
@@ -79,7 +79,7 @@ public function testGetEffectiveStartEndUnbound(): void
 VCAL;
 
         $document = Reader::read($vcal);
-        $this->assertEquals(
+        self::assertEquals(
             [
                 null,
                 null,
@@ -98,7 +98,7 @@ public function testIsInTimeRangeUnbound(): void
 VCAL;
 
         $document = Reader::read($vcal);
-        $this->assertTrue(
+        self::assertTrue(
             $document->VAVAILABILITY->isInTimeRange(new \DateTimeImmutable('2015-07-17'), new \DateTimeImmutable('2015-07-18'))
         );
     }
@@ -115,7 +115,7 @@ public function testIsInTimeRangeOutside(): void
 VCAL;
 
         $document = Reader::read($vcal);
-        $this->assertFalse(
+        self::assertFalse(
             $document->VAVAILABILITY->isInTimeRange(new \DateTimeImmutable('2015-07-17'), new \DateTimeImmutable('2015-07-18'))
         );
     }
@@ -123,7 +123,7 @@ public function testIsInTimeRangeOutside(): void
     public function testRFCxxxSection3Part1AvailabilityPropRequired(): void
     {
         // UID and DTSTAMP are present.
-        $this->assertIsValid(Reader::read(
+        self::assertIsValid(Reader::read(
             <<<VCAL
 BEGIN:VCALENDAR
 VERSION:2.0
@@ -137,7 +137,7 @@ public function testRFCxxxSection3Part1AvailabilityPropRequired(): void
         ));
 
         // UID and DTSTAMP are missing.
-        $this->assertIsNotValid(Reader::read(
+        self::assertIsNotValid(Reader::read(
             <<<VCAL
 BEGIN:VCALENDAR
 VERSION:2.0
@@ -149,7 +149,7 @@ public function testRFCxxxSection3Part1AvailabilityPropRequired(): void
         ));
 
         // DTSTAMP is missing.
-        $this->assertIsNotValid(Reader::read(
+        self::assertIsNotValid(Reader::read(
             <<<VCAL
 BEGIN:VCALENDAR
 VERSION:2.0
@@ -162,7 +162,7 @@ public function testRFCxxxSection3Part1AvailabilityPropRequired(): void
         ));
 
         // UID is missing.
-        $this->assertIsNotValid(Reader::read(
+        self::assertIsNotValid(Reader::read(
             <<<VCAL
 BEGIN:VCALENDAR
 VERSION:2.0
@@ -192,11 +192,11 @@ public function testRFCxxxSection3Part1AvailabilityPropOptionalOnce(): void
         ];
 
         // They are all present, only once.
-        $this->assertIsValid(Reader::read($this->template($properties)));
+        self::assertIsValid(Reader::read($this->template($properties)));
 
         // We duplicate each one to see if it fails.
         foreach ($properties as $property) {
-            $this->assertIsNotValid(Reader::read($this->template([
+            self::assertIsNotValid(Reader::read($this->template([
                 $property,
                 $property,
             ])));
@@ -206,17 +206,17 @@ public function testRFCxxxSection3Part1AvailabilityPropOptionalOnce(): void
     public function testRFCxxxSection3Part1AvailabilityPropDtendDuration(): void
     {
         // Only DTEND.
-        $this->assertIsValid(Reader::read($this->template([
+        self::assertIsValid(Reader::read($this->template([
             'DTEND:21111005T133225Z',
         ])));
 
         // Only DURATION.
-        $this->assertIsValid(Reader::read($this->template([
+        self::assertIsValid(Reader::read($this->template([
             'DURATION:PT1H',
         ])));
 
         // Both (not allowed).
-        $this->assertIsNotValid(Reader::read($this->template([
+        self::assertIsNotValid(Reader::read($this->template([
             'DTEND:21111005T133225Z',
             'DURATION:PT1H',
         ])));
@@ -234,13 +234,13 @@ public function testAvailableSubComponent(): void
 VCAL;
         $document = Reader::read($vcal);
 
-        $this->assertInstanceOf(Available::class, $document->VAVAILABILITY->AVAILABLE);
+        self::assertInstanceOf(Available::class, $document->VAVAILABILITY->AVAILABLE);
     }
 
     public function testRFCxxxSection3Part1AvailablePropRequired(): void
     {
         // UID, DTSTAMP and DTSTART are present.
-        $this->assertIsValid(Reader::read(
+        self::assertIsValid(Reader::read(
             <<<VCAL
 BEGIN:VCALENDAR
 VERSION:2.0
@@ -259,7 +259,7 @@ public function testRFCxxxSection3Part1AvailablePropRequired(): void
         ));
 
         // UID, DTSTAMP and DTSTART are missing.
-        $this->assertIsNotValid(Reader::read(
+        self::assertIsNotValid(Reader::read(
             <<<VCAL
 BEGIN:VCALENDAR
 VERSION:2.0
@@ -275,7 +275,7 @@ public function testRFCxxxSection3Part1AvailablePropRequired(): void
         ));
 
         // UID is missing.
-        $this->assertIsNotValid(Reader::read(
+        self::assertIsNotValid(Reader::read(
             <<<VCAL
 BEGIN:VCALENDAR
 VERSION:2.0
@@ -293,7 +293,7 @@ public function testRFCxxxSection3Part1AvailablePropRequired(): void
         ));
 
         // DTSTAMP is missing.
-        $this->assertIsNotValid(Reader::read(
+        self::assertIsNotValid(Reader::read(
             <<<VCAL
 BEGIN:VCALENDAR
 VERSION:2.0
@@ -311,7 +311,7 @@ public function testRFCxxxSection3Part1AvailablePropRequired(): void
         ));
 
         // DTSTART is missing.
-        $this->assertIsNotValid(Reader::read(
+        self::assertIsNotValid(Reader::read(
             <<<VCAL
 BEGIN:VCALENDAR
 VERSION:2.0
@@ -332,17 +332,17 @@ public function testRFCxxxSection3Part1AvailablePropRequired(): void
     public function testRFCxxxSection3Part1AvailableDtendDuration(): void
     {
         // Only DTEND.
-        $this->assertIsValid(Reader::read($this->templateAvailable([
+        self::assertIsValid(Reader::read($this->templateAvailable([
             'DTEND:21111005T133225Z',
         ])));
 
         // Only DURATION.
-        $this->assertIsValid(Reader::read($this->templateAvailable([
+        self::assertIsValid(Reader::read($this->templateAvailable([
             'DURATION:PT1H',
         ])));
 
         // Both (not allowed).
-        $this->assertIsNotValid(Reader::read($this->templateAvailable([
+        self::assertIsNotValid(Reader::read($this->templateAvailable([
             'DTEND:21111005T133225Z',
             'DURATION:PT1H',
         ])));
@@ -360,11 +360,11 @@ public function testRFCxxxSection3Part1AvailableOptionalOnce(): void
         ];
 
         // They are all present, only once.
-        $this->assertIsValid(Reader::read($this->templateAvailable($properties)));
+        self::assertIsValid(Reader::read($this->templateAvailable($properties)));
 
         // We duplicate each one to see if it fails.
         foreach ($properties as $property) {
-            $this->assertIsNotValid(Reader::read($this->templateAvailable([
+            self::assertIsNotValid(Reader::read($this->templateAvailable([
                 $property,
                 $property,
             ])));
@@ -373,7 +373,7 @@ public function testRFCxxxSection3Part1AvailableOptionalOnce(): void
 
     public function testRFCxxxSection3Part2(): void
     {
-        $this->assertEquals(
+        self::assertEquals(
             'BUSY',
             Reader::read($this->templateAvailable([
                 'BUSYTYPE:BUSY',
@@ -384,7 +384,7 @@ public function testRFCxxxSection3Part2(): void
                 ->getValue()
         );
 
-        $this->assertEquals(
+        self::assertEquals(
             'BUSY-UNAVAILABLE',
             Reader::read($this->templateAvailable([
                 'BUSYTYPE:BUSY-UNAVAILABLE',
@@ -395,7 +395,7 @@ public function testRFCxxxSection3Part2(): void
                 ->getValue()
         );
 
-        $this->assertEquals(
+        self::assertEquals(
             'BUSY-TENTATIVE',
             Reader::read($this->templateAvailable([
                 'BUSYTYPE:BUSY-TENTATIVE',
@@ -414,12 +414,12 @@ protected function assertIsValid(VObject\Document $document): void
             $messages = array_map(function ($item) { return $item['message']; }, $validationResult);
             $this->fail('Failed to assert that the supplied document is a valid document. Validation messages: '.implode(', ', $messages));
         }
-        $this->assertEmpty($document->validate());
+        self::assertEmpty($document->validate());
     }
 
     protected function assertIsNotValid(VObject\Document $document): void
     {
-        $this->assertNotEmpty($document->validate());
+        self::assertNotEmpty($document->validate());
     }
 
     protected function template(array $properties)
diff --git a/tests/VObject/Component/VCalendarTest.php b/tests/VObject/Component/VCalendarTest.php
index d1a2e540..bde6a4fa 100644
--- a/tests/VObject/Component/VCalendarTest.php
+++ b/tests/VObject/Component/VCalendarTest.php
@@ -28,7 +28,7 @@ public function testExpand(string $input, string $output, string $timeZone = 'UT
         // This will normalize the output
         $output = VObject\Reader::read($output)->serialize();
 
-        $this->assertVObjectEqualsVObject($output, $vcal->serialize());
+        self::assertVObjectEqualsVObject($output, $vcal->serialize());
     }
 
     public function expandData(): array
@@ -373,14 +373,14 @@ public function testEventExpandYearly(): void
             new \DateTime('2023-01-01')
         );
 
-        $this->assertCount(7, $events->VEVENT);
+        self::assertCount(7, $events->VEVENT);
     }
 
     public function testGetDocumentType(): void
     {
         $vcard = new VCalendar();
         $vcard->VERSION = '2.0';
-        $this->assertEquals(VCalendar::ICALENDAR20, $vcard->getDocumentType());
+        self::assertEquals(VCalendar::ICALENDAR20, $vcard->getDocumentType());
     }
 
     public function testValidateCorrect(): void
@@ -398,7 +398,7 @@ public function testValidateCorrect(): void
 ';
 
         $vcal = VObject\Reader::read($input);
-        $this->assertEquals([], $vcal->validate(), 'Got an error');
+        self::assertEquals([], $vcal->validate(), 'Got an error');
     }
 
     public function testValidateNoVersion(): void
@@ -415,7 +415,7 @@ public function testValidateNoVersion(): void
 ';
 
         $vcal = VObject\Reader::read($input);
-        $this->assertCount(1, $vcal->validate());
+        self::assertCount(1, $vcal->validate());
     }
 
     public function testValidateWrongVersion(): void
@@ -433,7 +433,7 @@ public function testValidateWrongVersion(): void
 ';
 
         $vcal = VObject\Reader::read($input);
-        $this->assertCount(1, $vcal->validate());
+        self::assertCount(1, $vcal->validate());
     }
 
     public function testValidateNoProdId(): void
@@ -450,7 +450,7 @@ public function testValidateNoProdId(): void
 ';
 
         $vcal = VObject\Reader::read($input);
-        $this->assertCount(1, $vcal->validate());
+        self::assertCount(1, $vcal->validate());
     }
 
     public function testValidateDoubleCalScale(): void
@@ -469,7 +469,7 @@ public function testValidateDoubleCalScale(): void
 ';
 
         $vcal = VObject\Reader::read($input);
-        $this->assertCount(1, $vcal->validate());
+        self::assertCount(1, $vcal->validate());
     }
 
     public function testValidateDoubleMethod(): void
@@ -488,7 +488,7 @@ public function testValidateDoubleMethod(): void
 ';
 
         $vcal = VObject\Reader::read($input);
-        $this->assertCount(1, $vcal->validate());
+        self::assertCount(1, $vcal->validate());
     }
 
     public function testValidateTwoMasterEvents(): void
@@ -511,7 +511,7 @@ public function testValidateTwoMasterEvents(): void
 ';
 
         $vcal = VObject\Reader::read($input);
-        $this->assertCount(1, $vcal->validate());
+        self::assertCount(1, $vcal->validate());
     }
 
     public function testValidateOneMasterEvent(): void
@@ -535,7 +535,7 @@ public function testValidateOneMasterEvent(): void
 ';
 
         $vcal = VObject\Reader::read($input);
-        $this->assertCount(0, $vcal->validate());
+        self::assertCount(0, $vcal->validate());
     }
 
     public function testGetBaseComponent(): void
@@ -562,7 +562,7 @@ public function testGetBaseComponent(): void
         $vcal = VObject\Reader::read($input);
 
         $result = $vcal->getBaseComponent();
-        $this->assertEquals('test', $result->SUMMARY->getValue());
+        self::assertEquals('test', $result->SUMMARY->getValue());
     }
 
     public function testGetBaseComponentNoResult(): void
@@ -590,7 +590,7 @@ public function testGetBaseComponentNoResult(): void
         $vcal = VObject\Reader::read($input);
 
         $result = $vcal->getBaseComponent();
-        $this->assertNull($result);
+        self::assertNull($result);
     }
 
     public function testGetBaseComponentWithFilter(): void
@@ -617,7 +617,7 @@ public function testGetBaseComponentWithFilter(): void
         $vcal = VObject\Reader::read($input);
 
         $result = $vcal->getBaseComponent('VEVENT');
-        $this->assertEquals('test', $result->SUMMARY->getValue());
+        self::assertEquals('test', $result->SUMMARY->getValue());
     }
 
     public function testGetBaseComponentWithFilterNoResult(): void
@@ -637,7 +637,7 @@ public function testGetBaseComponentWithFilterNoResult(): void
         $vcal = VObject\Reader::read($input);
 
         $result = $vcal->getBaseComponent('VEVENT');
-        $this->assertNull($result);
+        self::assertNull($result);
     }
 
     public function testNoComponents(): void
@@ -649,7 +649,7 @@ public function testNoComponents(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertValidate(
+        self::assertValidate(
             $input,
             0,
             3,
@@ -669,7 +669,7 @@ public function testCalDAVNoComponents(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertValidate(
+        self::assertValidate(
             $input,
             VCalendar::PROFILE_CALDAV,
             3,
@@ -696,7 +696,7 @@ public function testCalDAVMultiUID(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertValidate(
+        self::assertValidate(
             $input,
             VCalendar::PROFILE_CALDAV,
             3,
@@ -724,7 +724,7 @@ public function testCalDAVMultiComponent(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertValidate(
+        self::assertValidate(
             $input,
             VCalendar::PROFILE_CALDAV,
             3,
@@ -748,7 +748,7 @@ public function testCalDAVMETHOD(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertValidate(
+        self::assertValidate(
             $input,
             VCalendar::PROFILE_CALDAV,
             3,
@@ -761,7 +761,7 @@ public function assertValidate($ics, $options, $expectedLevel, ?string $expected
         $vcal = VObject\Reader::read($ics);
         $result = $vcal->validate($options);
 
-        $this->assertValidateResult($result, $expectedLevel, $expectedMessage);
+        self::assertValidateResult($result, $expectedLevel, $expectedMessage);
     }
 
     public function assertValidateResult($input, $expectedLevel, ?string $expectedMessage = null): void
@@ -772,12 +772,12 @@ public function assertValidateResult($input, $expectedLevel, ?string $expectedMe
         }
 
         if (0 === $expectedLevel) {
-            $this->assertCount(0, $input, 'No validation messages were expected. We got: '.implode(', ', $messages));
+            self::assertCount(0, $input, 'No validation messages were expected. We got: '.implode(', ', $messages));
         } else {
-            $this->assertCount(1, $input, 'We expected exactly 1 validation message, We got: '.implode(', ', $messages));
+            self::assertCount(1, $input, 'We expected exactly 1 validation message, We got: '.implode(', ', $messages));
 
-            $this->assertEquals($expectedMessage, $input[0]['message']);
-            $this->assertEquals($expectedLevel, $input[0]['level']);
+            self::assertEquals($expectedMessage, $input[0]['message']);
+            self::assertEquals($expectedLevel, $input[0]['level']);
         }
     }
 }
diff --git a/tests/VObject/Component/VCardTest.php b/tests/VObject/Component/VCardTest.php
index da15de1c..3851d97d 100644
--- a/tests/VObject/Component/VCardTest.php
+++ b/tests/VObject/Component/VCardTest.php
@@ -21,11 +21,11 @@ public function testValidate(string $input, array $expectedWarnings, string $exp
             $warnMsg[] = $warning['message'];
         }
 
-        $this->assertEquals($expectedWarnings, $warnMsg);
+        self::assertEquals($expectedWarnings, $warnMsg);
 
         $vcard->validate(VObject\Component::REPAIR);
 
-        $this->assertEquals(
+        self::assertEquals(
             $expectedRepairedOutput,
             $vcard->serialize()
         );
@@ -116,18 +116,18 @@ public function testGetDocumentType(): void
     {
         $vcard = new VCard([], false);
         $vcard->VERSION = '2.1';
-        $this->assertEquals(VCard::VCARD21, $vcard->getDocumentType());
+        self::assertEquals(VCard::VCARD21, $vcard->getDocumentType());
 
         $vcard = new VCard([], false);
         $vcard->VERSION = '3.0';
-        $this->assertEquals(VCard::VCARD30, $vcard->getDocumentType());
+        self::assertEquals(VCard::VCARD30, $vcard->getDocumentType());
 
         $vcard = new VCard([], false);
         $vcard->VERSION = '4.0';
-        $this->assertEquals(VCard::VCARD40, $vcard->getDocumentType());
+        self::assertEquals(VCard::VCARD40, $vcard->getDocumentType());
 
         $vcard = new VCard([], false);
-        $this->assertEquals(VCard::UNKNOWN, $vcard->getDocumentType());
+        self::assertEquals(VCard::UNKNOWN, $vcard->getDocumentType());
     }
 
     public function testGetByType(): void
@@ -141,10 +141,10 @@ public function testGetByType(): void
 VCF;
 
         $vcard = VObject\Reader::read($vcard);
-        $this->assertEquals('1@example.org', $vcard->getByType('EMAIL', 'home')->getValue());
-        $this->assertEquals('2@example.org', $vcard->getByType('EMAIL', 'work')->getValue());
-        $this->assertNull($vcard->getByType('EMAIL', 'non-existent'));
-        $this->assertNull($vcard->getByType('ADR', 'non-existent'));
+        self::assertEquals('1@example.org', $vcard->getByType('EMAIL', 'home')->getValue());
+        self::assertEquals('2@example.org', $vcard->getByType('EMAIL', 'work')->getValue());
+        self::assertNull($vcard->getByType('EMAIL', 'non-existent'));
+        self::assertNull($vcard->getByType('ADR', 'non-existent'));
     }
 
     public function testPreferredNoPref(): void
@@ -158,7 +158,7 @@ public function testPreferredNoPref(): void
 VCF;
 
         $vcard = VObject\Reader::read($vcard);
-        $this->assertEquals('1@example.org', $vcard->preferred('EMAIL')->getValue());
+        self::assertEquals('1@example.org', $vcard->preferred('EMAIL')->getValue());
     }
 
     public function testPreferredWithPref(): void
@@ -172,7 +172,7 @@ public function testPreferredWithPref(): void
 VCF;
 
         $vcard = VObject\Reader::read($vcard);
-        $this->assertEquals('2@example.org', $vcard->preferred('EMAIL')->getValue());
+        self::assertEquals('2@example.org', $vcard->preferred('EMAIL')->getValue());
     }
 
     public function testPreferredWith40Pref(): void
@@ -187,7 +187,7 @@ public function testPreferredWith40Pref(): void
 VCF;
 
         $vcard = VObject\Reader::read($vcard);
-        $this->assertEquals('3@example.org', $vcard->preferred('EMAIL')->getValue());
+        self::assertEquals('3@example.org', $vcard->preferred('EMAIL')->getValue());
     }
 
     public function testPreferredNotFound(): void
@@ -199,7 +199,7 @@ public function testPreferredNotFound(): void
 VCF;
 
         $vcard = VObject\Reader::read($vcard);
-        $this->assertNull($vcard->preferred('EMAIL'));
+        self::assertNull($vcard->preferred('EMAIL'));
     }
 
     public function testNoUIDCardDAV(): void
@@ -210,7 +210,7 @@ public function testNoUIDCardDAV(): void
 FN:John Doe
 END:VCARD
 VCF;
-        $this->assertValidate(
+        self::assertValidate(
             $vcard,
             VCard::PROFILE_CARDDAV,
             3,
@@ -226,7 +226,7 @@ public function testNoUIDNoCardDAV(): void
 FN:John Doe
 END:VCARD
 VCF;
-        $this->assertValidate(
+        self::assertValidate(
             $vcard,
             0,
             2,
@@ -242,7 +242,7 @@ public function testNoUIDNoCardDAVRepair(): void
 FN:John Doe
 END:VCARD
 VCF;
-        $this->assertValidate(
+        self::assertValidate(
             $vcard,
             VCard::REPAIR,
             1,
@@ -259,7 +259,7 @@ public function testVCard21CardDAV(): void
 UID:foo
 END:VCARD
 VCF;
-        $this->assertValidate(
+        self::assertValidate(
             $vcard,
             VCard::PROFILE_CARDDAV,
             3,
@@ -276,7 +276,7 @@ public function testVCard21NoCardDAV(): void
 UID:foo
 END:VCARD
 VCF;
-        $this->assertValidate(
+        self::assertValidate(
             $vcard,
             0,
             0
@@ -288,7 +288,7 @@ public function assertValidate($vcf, $options, int $expectedLevel, ?string $expe
         $vcal = VObject\Reader::read($vcf);
         $result = $vcal->validate($options);
 
-        $this->assertValidateResult($result, $expectedLevel, $expectedMessage);
+        self::assertValidateResult($result, $expectedLevel, $expectedMessage);
     }
 
     public function assertValidateResult($input, int $expectedLevel, ?string $expectedMessage = null): void
@@ -299,12 +299,12 @@ public function assertValidateResult($input, int $expectedLevel, ?string $expect
         }
 
         if (0 === $expectedLevel) {
-            $this->assertCount(0, $input, 'No validation messages were expected. We got: '.implode(', ', $messages));
+            self::assertCount(0, $input, 'No validation messages were expected. We got: '.implode(', ', $messages));
         } else {
-            $this->assertCount(1, $input, 'We expected exactly 1 validation message, We got: '.implode(', ', $messages));
+            self::assertCount(1, $input, 'We expected exactly 1 validation message, We got: '.implode(', ', $messages));
 
-            $this->assertEquals($expectedMessage, $input[0]['message']);
-            $this->assertEquals($expectedLevel, $input[0]['level']);
+            self::assertEquals($expectedMessage, $input[0]['message']);
+            self::assertEquals($expectedLevel, $input[0]['level']);
         }
     }
 }
diff --git a/tests/VObject/Component/VEventTest.php b/tests/VObject/Component/VEventTest.php
index 8d73ac11..50fa626c 100644
--- a/tests/VObject/Component/VEventTest.php
+++ b/tests/VObject/Component/VEventTest.php
@@ -11,7 +11,7 @@ class VEventTest extends TestCase
      */
     public function testInTimeRange(VEvent $vevent, \DateTime $start, \DateTime $end, bool $outcome): void
     {
-        $this->assertEquals($outcome, $vevent->isInTimeRange($start, $end));
+        self::assertEquals($outcome, $vevent->isInTimeRange($start, $end));
     }
 
     public function timeRangeTestData(): array
diff --git a/tests/VObject/Component/VFreeBusyTest.php b/tests/VObject/Component/VFreeBusyTest.php
index c1e22dd1..f3a521f9 100644
--- a/tests/VObject/Component/VFreeBusyTest.php
+++ b/tests/VObject/Component/VFreeBusyTest.php
@@ -28,14 +28,14 @@ public function testIsFree(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertFalse($vfb->isFree(new \DateTime('2012-09-12 01:15:00', $tz), new \DateTime('2012-09-12 01:45:00', $tz)));
-        $this->assertFalse($vfb->isFree(new \DateTime('2012-09-12 08:05:00', $tz), new \DateTime('2012-09-12 08:10:00', $tz)));
-        $this->assertFalse($vfb->isFree(new \DateTime('2012-09-12 10:15:00', $tz), new \DateTime('2012-09-12 10:45:00', $tz)));
+        self::assertFalse($vfb->isFree(new \DateTime('2012-09-12 01:15:00', $tz), new \DateTime('2012-09-12 01:45:00', $tz)));
+        self::assertFalse($vfb->isFree(new \DateTime('2012-09-12 08:05:00', $tz), new \DateTime('2012-09-12 08:10:00', $tz)));
+        self::assertFalse($vfb->isFree(new \DateTime('2012-09-12 10:15:00', $tz), new \DateTime('2012-09-12 10:45:00', $tz)));
 
         // Checking whether the end time is treated as non-inclusive
-        $this->assertTrue($vfb->isFree(new \DateTime('2012-09-12 09:00:00', $tz), new \DateTime('2012-09-12 09:15:00', $tz)));
-        $this->assertTrue($vfb->isFree(new \DateTime('2012-09-12 09:45:00', $tz), new \DateTime('2012-09-12 10:00:00', $tz)));
-        $this->assertTrue($vfb->isFree(new \DateTime('2012-09-12 11:00:00', $tz), new \DateTime('2012-09-12 12:00:00', $tz)));
+        self::assertTrue($vfb->isFree(new \DateTime('2012-09-12 09:00:00', $tz), new \DateTime('2012-09-12 09:15:00', $tz)));
+        self::assertTrue($vfb->isFree(new \DateTime('2012-09-12 09:45:00', $tz), new \DateTime('2012-09-12 10:00:00', $tz)));
+        self::assertTrue($vfb->isFree(new \DateTime('2012-09-12 11:00:00', $tz), new \DateTime('2012-09-12 12:00:00', $tz)));
     }
 
     public function testValidate(): void
@@ -59,6 +59,6 @@ public function testValidate(): void
             $messages[] = $warning['message'];
         }
 
-        $this->assertEquals([], $messages);
+        self::assertEquals([], $messages);
     }
 }
diff --git a/tests/VObject/Component/VJournalTest.php b/tests/VObject/Component/VJournalTest.php
index 012c9a31..98f47889 100644
--- a/tests/VObject/Component/VJournalTest.php
+++ b/tests/VObject/Component/VJournalTest.php
@@ -12,7 +12,7 @@ class VJournalTest extends TestCase
      */
     public function testInTimeRange(VJournal $vtodo, \DateTime $start, \DateTime $end, bool $outcome): void
     {
-        $this->assertEquals($outcome, $vtodo->isInTimeRange($start, $end));
+        self::assertEquals($outcome, $vtodo->isInTimeRange($start, $end));
     }
 
     public function testValidate(): void
@@ -36,7 +36,7 @@ public function testValidate(): void
             $messages[] = $warning['message'];
         }
 
-        $this->assertEquals([], $messages);
+        self::assertEquals([], $messages);
     }
 
     public function testValidateBroken(): void
@@ -62,7 +62,7 @@ public function testValidateBroken(): void
             $messages[] = $warning['message'];
         }
 
-        $this->assertEquals(
+        self::assertEquals(
             ['URL MUST NOT appear more than once in a VJOURNAL component'],
             $messages
         );
diff --git a/tests/VObject/Component/VTimeZoneTest.php b/tests/VObject/Component/VTimeZoneTest.php
index 9b8ab99e..2928ecd0 100644
--- a/tests/VObject/Component/VTimeZoneTest.php
+++ b/tests/VObject/Component/VTimeZoneTest.php
@@ -27,7 +27,7 @@ public function testValidate(): void
             $messages[] = $warning['message'];
         }
 
-        $this->assertEquals([], $messages);
+        self::assertEquals([], $messages);
     }
 
     public function testGetTimeZone(): void
@@ -46,7 +46,7 @@ public function testGetTimeZone(): void
 
         $tz = new \DateTimeZone('America/Toronto');
 
-        $this->assertEquals(
+        self::assertEquals(
             $tz,
             $obj->VTIMEZONE->getTimeZone()
         );
diff --git a/tests/VObject/Component/VTodoTest.php b/tests/VObject/Component/VTodoTest.php
index ae0050aa..808675b3 100644
--- a/tests/VObject/Component/VTodoTest.php
+++ b/tests/VObject/Component/VTodoTest.php
@@ -12,7 +12,7 @@ class VTodoTest extends TestCase
      */
     public function testInTimeRange(VTodo $vtodo, \DateTime $start, \DateTime $end, bool $outcome): void
     {
-        $this->assertEquals($outcome, $vtodo->isInTimeRange($start, $end));
+        self::assertEquals($outcome, $vtodo->isInTimeRange($start, $end));
     }
 
     public function timeRangeTestData(): array
@@ -85,7 +85,7 @@ public function testValidate(): void
             $messages[] = $warning['message'];
         }
 
-        $this->assertEquals([], $messages);
+        self::assertEquals([], $messages);
     }
 
     public function testValidateInvalid(): void
@@ -107,7 +107,7 @@ public function testValidateInvalid(): void
             $messages[] = $warning['message'];
         }
 
-        $this->assertEquals([
+        self::assertEquals([
             'UID MUST appear exactly once in a VTODO component',
             'DTSTAMP MUST appear exactly once in a VTODO component',
         ], $messages);
@@ -136,7 +136,7 @@ public function testValidateDueDateTimeStartMisMatch(): void
             $messages[] = $warning['message'];
         }
 
-        $this->assertEquals([
+        self::assertEquals([
             'The value type (DATE or DATE-TIME) must be identical for DUE and DTSTART',
         ], $messages);
     }
@@ -164,7 +164,7 @@ public function testValidateDueBeforeDateTimeStart(): void
             $messages[] = $warning['message'];
         }
 
-        $this->assertEquals([
+        self::assertEquals([
             'DUE must occur after DTSTART',
         ], $messages);
     }
diff --git a/tests/VObject/ComponentTest.php b/tests/VObject/ComponentTest.php
index 9bb680b7..4a247d90 100644
--- a/tests/VObject/ComponentTest.php
+++ b/tests/VObject/ComponentTest.php
@@ -21,13 +21,13 @@ public function testIterate(): void
         $count = 0;
         foreach ($comp->children() as $key => $subcomponent) {
             ++$count;
-            $this->assertInstanceOf(Component::class, $subcomponent);
+            self::assertInstanceOf(Component::class, $subcomponent);
 
             if (2 === $count) {
-                $this->assertEquals(1, $key);
+                self::assertEquals(1, $key);
             }
         }
-        $this->assertEquals(2, $count);
+        self::assertEquals(2, $count);
     }
 
     public function testMagicGet(): void
@@ -41,10 +41,10 @@ public function testMagicGet(): void
         $comp->add($sub);
 
         $event = $comp->VEVENT;
-        $this->assertInstanceOf(Component::class, $event);
-        $this->assertEquals('VEVENT', $event->name);
+        self::assertInstanceOf(Component::class, $event);
+        self::assertEquals('VEVENT', $event->name);
 
-        $this->assertNull($comp->VJOURNAL);
+        self::assertNull($comp->VJOURNAL);
     }
 
     /**
@@ -67,26 +67,26 @@ public function testMagicGetGroups(): void
         $comp->add($sub);
 
         $emails = $comp->EMAIL;
-        $this->assertCount(4, $emails);
+        self::assertCount(4, $emails);
 
         $email1 = $comp->{'group1.email'};
-        $this->assertEquals('EMAIL', $email1[0]->name);
-        $this->assertEquals('GROUP1', $email1[0]->group);
+        self::assertEquals('EMAIL', $email1[0]->name);
+        self::assertEquals('GROUP1', $email1[0]->group);
 
         $email0 = $comp->{'0.email'};
-        $this->assertEquals('EMAIL', $email0[0]->name);
-        $this->assertEquals('0', $email0[0]->group);
+        self::assertEquals('EMAIL', $email0[0]->name);
+        self::assertEquals('0', $email0[0]->group);
 
         // this is supposed to return all EMAIL properties that do not have a group
         $email3 = $comp->{'.email'};
-        $this->assertEquals('EMAIL', $email3[0]->name);
-        $this->assertEquals(null, $email3[0]->group);
+        self::assertEquals('EMAIL', $email3[0]->name);
+        self::assertEquals(null, $email3[0]->group);
 
         // this is supposed to return all properties that do not have a group
         $nogroupProps = $comp->{'.'};
-        $this->assertGreaterThan(0, count($email3));
+        self::assertGreaterThan(0, count($email3));
         foreach ($nogroupProps as $prop) {
-            $this->assertEquals(null, $prop->group);
+            self::assertEquals(null, $prop->group);
         }
     }
 
@@ -101,9 +101,9 @@ public function testAddGroupProperties(): void
 
         foreach (['item2', 'ITEM1'] as $group) {
             $prop = $comp->{"$group.X-ABLabel"};
-            $this->assertInstanceOf(Property::class, $prop);
-            $this->assertSame("$group-Foo", (string) $prop);
-            $this->assertSame($group, $prop->group);
+            self::assertInstanceOf(Property::class, $prop);
+            self::assertSame("$group-Foo", (string) $prop);
+            self::assertSame($group, $prop->group);
         }
     }
 
@@ -117,9 +117,9 @@ public function testMagicIsset(): void
         $sub = $comp->createComponent('VTODO');
         $comp->add($sub);
 
-        $this->assertTrue(isset($comp->vevent));
-        $this->assertTrue(isset($comp->vtodo));
-        $this->assertFalse(isset($comp->vjournal));
+        self::assertTrue(isset($comp->vevent));
+        self::assertTrue(isset($comp->vtodo));
+        self::assertFalse(isset($comp->vjournal));
     }
 
     public function testMagicSetScalar(): void
@@ -127,8 +127,8 @@ public function testMagicSetScalar(): void
         $comp = new VCalendar();
         $comp->myProp = 'myValue';
 
-        $this->assertInstanceOf(Property::class, $comp->MYPROP);
-        $this->assertEquals('myValue', (string) $comp->MYPROP);
+        self::assertInstanceOf(Property::class, $comp->MYPROP);
+        self::assertEquals('myValue', (string) $comp->MYPROP);
     }
 
     public function testMagicSetScalarTwice(): void
@@ -137,9 +137,9 @@ public function testMagicSetScalarTwice(): void
         $comp->myProp = 'myValue';
         $comp->myProp = 'myValue';
 
-        $this->assertCount(1, $comp->children());
-        $this->assertInstanceOf(Property::class, $comp->MYPROP);
-        $this->assertEquals('myValue', (string) $comp->MYPROP);
+        self::assertCount(1, $comp->children());
+        self::assertInstanceOf(Property::class, $comp->MYPROP);
+        self::assertEquals('myValue', (string) $comp->MYPROP);
     }
 
     public function testMagicSetArray(): void
@@ -147,8 +147,8 @@ public function testMagicSetArray(): void
         $comp = new VCalendar();
         $comp->ORG = ['Acme Inc', 'Section 9'];
 
-        $this->assertInstanceOf(Property::class, $comp->ORG);
-        $this->assertEquals(['Acme Inc', 'Section 9'], $comp->ORG->getParts());
+        self::assertInstanceOf(Property::class, $comp->ORG);
+        self::assertEquals(['Acme Inc', 'Section 9'], $comp->ORG->getParts());
     }
 
     public function testMagicSetComponent(): void
@@ -158,9 +158,9 @@ public function testMagicSetComponent(): void
         // Note that 'myProp' is ignored here.
         $comp->myProp = $comp->createComponent('VEVENT');
 
-        $this->assertCount(1, $comp);
+        self::assertCount(1, $comp);
 
-        $this->assertEquals('VEVENT', $comp->VEVENT->name);
+        self::assertEquals('VEVENT', $comp->VEVENT->name);
     }
 
     public function testMagicSetTwice(): void
@@ -170,9 +170,9 @@ public function testMagicSetTwice(): void
         $comp->VEVENT = $comp->createComponent('VEVENT');
         $comp->VEVENT = $comp->createComponent('VEVENT');
 
-        $this->assertCount(1, $comp->children());
+        self::assertCount(1, $comp->children());
 
-        $this->assertEquals('VEVENT', $comp->VEVENT->name);
+        self::assertEquals('VEVENT', $comp->VEVENT->name);
     }
 
     public function testArrayAccessGet(): void
@@ -189,9 +189,9 @@ public function testArrayAccessGet(): void
 
         $comp->add($event2);
 
-        $this->assertCount(2, $comp->children());
-        $this->assertTrue($comp->vevent[1] instanceof Component);
-        $this->assertEquals('Event 2', (string) $comp->vevent[1]->summary);
+        self::assertCount(2, $comp->children());
+        self::assertTrue($comp->vevent[1] instanceof Component);
+        self::assertEquals('Event 2', (string) $comp->vevent[1]->summary);
     }
 
     public function testArrayAccessExists(): void
@@ -208,8 +208,8 @@ public function testArrayAccessExists(): void
 
         $comp->add($event2);
 
-        $this->assertTrue(isset($comp->vevent[0]));
-        $this->assertTrue(isset($comp->vevent[1]));
+        self::assertTrue(isset($comp->vevent[0]));
+        self::assertTrue(isset($comp->vevent[1]));
     }
 
     public function testArrayAccessSet(): void
@@ -232,13 +232,13 @@ public function testAddScalar(): void
 
         $comp->add('myprop', 'value');
 
-        $this->assertCount(1, $comp->children());
+        self::assertCount(1, $comp->children());
 
         $bla = $comp->children()[0];
 
-        $this->assertTrue($bla instanceof Property);
-        $this->assertEquals('MYPROP', $bla->name);
-        $this->assertEquals('value', (string) $bla);
+        self::assertTrue($bla instanceof Property);
+        self::assertEquals('MYPROP', $bla->name);
+        self::assertEquals('value', (string) $bla);
     }
 
     public function testAddScalarParams(): void
@@ -247,18 +247,18 @@ public function testAddScalarParams(): void
 
         $comp->add('myprop', 'value', ['param1' => 'value1']);
 
-        $this->assertCount(1, $comp->children());
+        self::assertCount(1, $comp->children());
 
         $bla = $comp->children()[0];
 
-        $this->assertInstanceOf(Property::class, $bla);
-        $this->assertEquals('MYPROP', $bla->name);
-        $this->assertEquals('value', (string) $bla);
+        self::assertInstanceOf(Property::class, $bla);
+        self::assertEquals('MYPROP', $bla->name);
+        self::assertEquals('value', (string) $bla);
 
-        $this->assertCount(1, $bla->parameters());
+        self::assertCount(1, $bla->parameters());
 
-        $this->assertEquals('PARAM1', $bla->parameters['PARAM1']->name);
-        $this->assertEquals('value1', $bla->parameters['PARAM1']->getValue());
+        self::assertEquals('PARAM1', $bla->parameters['PARAM1']->name);
+        self::assertEquals('value1', $bla->parameters['PARAM1']->getValue());
     }
 
     public function testAddComponent(): void
@@ -267,9 +267,9 @@ public function testAddComponent(): void
 
         $comp->add($comp->createComponent('VEVENT'));
 
-        $this->assertCount(1, $comp->children());
+        self::assertCount(1, $comp->children());
 
-        $this->assertEquals('VEVENT', $comp->VEVENT->name);
+        self::assertEquals('VEVENT', $comp->VEVENT->name);
     }
 
     public function testAddComponentTwice(): void
@@ -279,9 +279,9 @@ public function testAddComponentTwice(): void
         $comp->add($comp->createComponent('VEVENT'));
         $comp->add($comp->createComponent('VEVENT'));
 
-        $this->assertCount(2, $comp->children());
+        self::assertCount(2, $comp->children());
 
-        $this->assertEquals('VEVENT', $comp->VEVENT->name);
+        self::assertEquals('VEVENT', $comp->VEVENT->name);
     }
 
     public function testAddArgFail(): void
@@ -305,13 +305,13 @@ public function testMagicUnset(): void
 
         unset($comp->vevent);
 
-        $this->assertCount(0, $comp->children());
+        self::assertCount(0, $comp->children());
     }
 
     public function testCount(): void
     {
         $comp = new VCalendar();
-        $this->assertEquals(1, $comp->count());
+        self::assertEquals(1, $comp->count());
     }
 
     public function testChildren(): void
@@ -323,8 +323,8 @@ public function testChildren(): void
         $comp->add($comp->createComponent('VTODO'));
 
         $r = $comp->children();
-        $this->assertIsArray($r);
-        $this->assertCount(2, $r);
+        self::assertIsArray($r);
+        self::assertCount(2, $r);
     }
 
     /**
@@ -338,15 +338,15 @@ public function testGetComponents(): void
         $comp->add($comp->createComponent('VTODO'));
 
         $r = $comp->getComponents();
-        $this->assertIsArray($r);
-        $this->assertCount(1, $r);
-        $this->assertEquals('VTODO', $r[0]->name);
+        self::assertIsArray($r);
+        self::assertCount(1, $r);
+        self::assertEquals('VTODO', $r[0]->name);
     }
 
     public function testSerialize(): void
     {
         $comp = new VCalendar([], false);
-        $this->assertEquals("BEGIN:VCALENDAR\r\nEND:VCALENDAR\r\n", $comp->serialize());
+        self::assertEquals("BEGIN:VCALENDAR\r\nEND:VCALENDAR\r\n", $comp->serialize());
     }
 
     public function testSerializeChildren(): void
@@ -359,7 +359,7 @@ public function testSerializeChildren(): void
 
         $str = $comp->serialize();
 
-        $this->assertEquals("BEGIN:VCALENDAR\r\nBEGIN:VEVENT\r\nEND:VEVENT\r\nBEGIN:VTODO\r\nEND:VTODO\r\nEND:VCALENDAR\r\n", $str);
+        self::assertEquals("BEGIN:VCALENDAR\r\nBEGIN:VEVENT\r\nEND:VEVENT\r\nBEGIN:VTODO\r\nEND:VTODO\r\nEND:VCALENDAR\r\n", $str);
     }
 
     public function testSerializeOrderCompAndProp(): void
@@ -373,7 +373,7 @@ public function testSerializeOrderCompAndProp(): void
         unset($event->DTSTAMP, $event->UID);
         $str = $comp->serialize();
 
-        $this->assertEquals("BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPROP1:BLABLA\r\nBEGIN:VTIMEZONE\r\nEND:VTIMEZONE\r\nBEGIN:VEVENT\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n", $str);
+        self::assertEquals("BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPROP1:BLABLA\r\nBEGIN:VTIMEZONE\r\nEND:VTIMEZONE\r\nBEGIN:VEVENT\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n", $str);
     }
 
     public function testAnotherSerializeOrderProp(): void
@@ -399,7 +399,7 @@ public function testAnotherSerializeOrderProp(): void
 
         $str = $comp->serialize();
 
-        $this->assertEquals("BEGIN:VCARD\r\nVERSION:2.0\r\nSOMEPROP:FOO\r\nANOTHERPROP:FOO\r\nTHIRDPROP:FOO\r\nPROP4:FOO 1\r\nPROP4:FOO 2\r\nPROP4:FOO 3\r\nPROP4:FOO 4\r\nPROP4:FOO 5\r\nPROP4:FOO 6\r\nPROP4:FOO 7\r\nPROP4:FOO 8\r\nPROP4:FOO 9\r\nPROP4:FOO 10\r\nPROPNUMBERFIVE:FOO\r\nPROPNUMBERSIX:FOO\r\nPROPNUMBERSEVEN:FOO\r\nPROPNUMBEREIGHT:FOO\r\nPROPNUMBERNINE:FOO\r\nPROPNUMBERTEN:FOO\r\nUID:FOO\r\nEND:VCARD\r\n", $str);
+        self::assertEquals("BEGIN:VCARD\r\nVERSION:2.0\r\nSOMEPROP:FOO\r\nANOTHERPROP:FOO\r\nTHIRDPROP:FOO\r\nPROP4:FOO 1\r\nPROP4:FOO 2\r\nPROP4:FOO 3\r\nPROP4:FOO 4\r\nPROP4:FOO 5\r\nPROP4:FOO 6\r\nPROP4:FOO 7\r\nPROP4:FOO 8\r\nPROP4:FOO 9\r\nPROP4:FOO 10\r\nPROPNUMBERFIVE:FOO\r\nPROPNUMBERSIX:FOO\r\nPROPNUMBERSEVEN:FOO\r\nPROPNUMBEREIGHT:FOO\r\nPROPNUMBERNINE:FOO\r\nPROPNUMBERTEN:FOO\r\nUID:FOO\r\nEND:VCARD\r\n", $str);
     }
 
     public function testInstantiateWithChildren(): void
@@ -409,8 +409,8 @@ public function testInstantiateWithChildren(): void
             'FN' => 'Finn The Human',
         ]);
 
-        $this->assertEquals(['Acme Inc.', 'Section 9'], $comp->ORG->getParts());
-        $this->assertEquals('Finn The Human', $comp->FN->getValue());
+        self::assertEquals(['Acme Inc.', 'Section 9'], $comp->ORG->getParts());
+        self::assertEquals('Finn The Human', $comp->FN->getValue());
     }
 
     public function testInstantiateSubComponent(): void
@@ -421,7 +421,7 @@ public function testInstantiateSubComponent(): void
         ]);
         $comp->add($event);
 
-        $this->assertEquals('12345', $comp->VEVENT->UID->getValue());
+        self::assertEquals('12345', $comp->VEVENT->UID->getValue());
     }
 
     public function testRemoveByName(): void
@@ -432,8 +432,8 @@ public function testRemoveByName(): void
         $comp->add('prop2', 'val2');
 
         $comp->remove('prop2');
-        $this->assertFalse(isset($comp->prop2));
-        $this->assertTrue(isset($comp->prop1));
+        self::assertFalse(isset($comp->prop2));
+        self::assertTrue(isset($comp->prop1));
     }
 
     public function testRemoveByObj(): void
@@ -443,8 +443,8 @@ public function testRemoveByObj(): void
         $prop = $comp->add('prop2', 'val2');
 
         $comp->remove($prop);
-        $this->assertFalse(isset($comp->prop2));
-        $this->assertTrue(isset($comp->prop1));
+        self::assertFalse(isset($comp->prop2));
+        self::assertTrue(isset($comp->prop1));
     }
 
     /**
@@ -470,7 +470,7 @@ public function testValidateRules(array $componentList, int $errorCount): void
             $component->add($v, 'Hello.');
         }
 
-        $this->assertCount($errorCount, $component->validate());
+        self::assertCount($errorCount, $component->validate());
     }
 
     public function testValidateRepair(): void
@@ -479,7 +479,7 @@ public function testValidateRepair(): void
 
         $component = new FakeComponent($vcard, 'Hi', [], false);
         $component->validate(Component::REPAIR);
-        $this->assertEquals('yow', $component->BAR->getValue());
+        self::assertEquals('yow', $component->BAR->getValue());
     }
 
     public function testValidateRepairShouldNotDeduplicatePropertiesWhenValuesDiffer(): void
@@ -493,9 +493,9 @@ public function testValidateRepairShouldNotDeduplicatePropertiesWhenValuesDiffer
 
         $messages = $component->validate(Component::REPAIR);
 
-        $this->assertCount(1, $messages);
-        $this->assertEquals(3, $messages[0]['level']);
-        $this->assertCount(2, $component->GIR);
+        self::assertCount(1, $messages);
+        self::assertEquals(3, $messages[0]['level']);
+        self::assertCount(2, $component->GIR);
     }
 
     public function testValidateRepairShouldNotDeduplicatePropertiesWhenParametersDiffer(): void
@@ -509,9 +509,9 @@ public function testValidateRepairShouldNotDeduplicatePropertiesWhenParametersDi
 
         $messages = $component->validate(Component::REPAIR);
 
-        $this->assertCount(1, $messages);
-        $this->assertEquals(3, $messages[0]['level']);
-        $this->assertCount(2, $component->GIR);
+        self::assertCount(1, $messages);
+        self::assertEquals(3, $messages[0]['level']);
+        self::assertCount(2, $component->GIR);
     }
 
     public function testValidateRepairShouldDeduplicatePropertiesWhenValuesAndParametersAreEqual(): void
@@ -525,9 +525,9 @@ public function testValidateRepairShouldDeduplicatePropertiesWhenValuesAndParame
 
         $messages = $component->validate(Component::REPAIR);
 
-        $this->assertCount(1, $messages);
-        $this->assertEquals(1, $messages[0]['level']);
-        $this->assertCount(1, $component->GIR);
+        self::assertCount(1, $messages);
+        self::assertEquals(1, $messages[0]['level']);
+        self::assertCount(1, $component->GIR);
     }
 
     public function testValidateRepairShouldDeduplicatePropertiesWhenValuesAreEqual(): void
@@ -541,9 +541,9 @@ public function testValidateRepairShouldDeduplicatePropertiesWhenValuesAreEqual(
 
         $messages = $component->validate(Component::REPAIR);
 
-        $this->assertCount(1, $messages);
-        $this->assertEquals(1, $messages[0]['level']);
-        $this->assertCount(1, $component->GIR);
+        self::assertCount(1, $messages);
+        self::assertEquals(1, $messages[0]['level']);
+        self::assertCount(1, $component->GIR);
     }
 
     public function ruleData(): array
diff --git a/tests/VObject/DateTimeParserTest.php b/tests/VObject/DateTimeParserTest.php
index a9612a14..b1e7f7cd 100644
--- a/tests/VObject/DateTimeParserTest.php
+++ b/tests/VObject/DateTimeParserTest.php
@@ -11,13 +11,13 @@ class DateTimeParserTest extends TestCase
      */
     public function testParseICalendarDuration(): void
     {
-        $this->assertEquals('+1 weeks', DateTimeParser::parseDurationAsString('P1W'));
-        $this->assertEquals('+5 days', DateTimeParser::parseDurationAsString('P5D'));
-        $this->assertEquals('+5 days 3 hours 50 minutes 12 seconds', DateTimeParser::parseDurationAsString('P5DT3H50M12S'));
-        $this->assertEquals('-1 weeks 50 minutes', DateTimeParser::parseDurationAsString('-P1WT50M'));
-        $this->assertEquals('+50 days 3 hours 2 seconds', DateTimeParser::parseDurationAsString('+P50DT3H2S'));
-        $this->assertEquals('+0 seconds', DateTimeParser::parseDurationAsString('+PT0S'));
-        $this->assertEquals(new \DateInterval('PT0S'), DateTimeParser::parseDuration('PT0S'));
+        self::assertEquals('+1 weeks', DateTimeParser::parseDurationAsString('P1W'));
+        self::assertEquals('+5 days', DateTimeParser::parseDurationAsString('P5D'));
+        self::assertEquals('+5 days 3 hours 50 minutes 12 seconds', DateTimeParser::parseDurationAsString('P5DT3H50M12S'));
+        self::assertEquals('-1 weeks 50 minutes', DateTimeParser::parseDurationAsString('-P1WT50M'));
+        self::assertEquals('+50 days 3 hours 2 seconds', DateTimeParser::parseDurationAsString('+P50DT3H2S'));
+        self::assertEquals('+0 seconds', DateTimeParser::parseDurationAsString('+PT0S'));
+        self::assertEquals(new \DateInterval('PT0S'), DateTimeParser::parseDuration('PT0S'));
     }
 
     /**
@@ -26,12 +26,12 @@ public function testParseICalendarDuration(): void
     public function testParseICalendarDurationDateInterval(): void
     {
         $expected = new \DateInterval('P7D');
-        $this->assertEquals($expected, DateTimeParser::parseDuration('P1W'));
-        $this->assertEquals($expected, DateTimeParser::parse('P1W'));
+        self::assertEquals($expected, DateTimeParser::parseDuration('P1W'));
+        self::assertEquals($expected, DateTimeParser::parse('P1W'));
 
         $expected = new \DateInterval('PT3M');
         $expected->invert = true;
-        $this->assertEquals($expected, DateTimeParser::parseDuration('-PT3M'));
+        self::assertEquals($expected, DateTimeParser::parseDuration('-PT3M'));
     }
 
     public function testParseICalendarDurationFail(): void
@@ -46,7 +46,7 @@ public function testParseICalendarDateTime(): void
 
         $compare = new \DateTimeImmutable('2010-03-16 14:14:05', new \DateTimeZone('UTC'));
 
-        $this->assertEquals($compare, $dateTime);
+        self::assertEquals($compare, $dateTime);
     }
 
     /**
@@ -75,7 +75,7 @@ public function testParseICalendarDateTimeUTC(): void
         $dateTime = DateTimeParser::parseDateTime('20100316T141405Z');
 
         $compare = new \DateTimeImmutable('2010-03-16 14:14:05', new \DateTimeZone('UTC'));
-        $this->assertEquals($compare, $dateTime);
+        self::assertEquals($compare, $dateTime);
     }
 
     /**
@@ -86,7 +86,7 @@ public function testParseICalendarDateTimeUTC2(): void
         $dateTime = DateTimeParser::parseDateTime('20101211T160000Z');
 
         $compare = new \DateTimeImmutable('2010-12-11 16:00:00', new \DateTimeZone('UTC'));
-        $this->assertEquals($compare, $dateTime);
+        self::assertEquals($compare, $dateTime);
     }
 
     /**
@@ -97,7 +97,7 @@ public function testParseICalendarDateTimeCustomTimeZone(): void
         $dateTime = DateTimeParser::parseDateTime('20100316T141405', new \DateTimeZone('Europe/Amsterdam'));
 
         $compare = new \DateTimeImmutable('2010-03-16 14:14:05', new \DateTimeZone('Europe/Amsterdam'));
-        $this->assertEquals($compare, $dateTime);
+        self::assertEquals($compare, $dateTime);
     }
 
     public function testParseICalendarDate(): void
@@ -106,10 +106,10 @@ public function testParseICalendarDate(): void
 
         $expected = new \DateTimeImmutable('2010-03-16 00:00:00', new \DateTimeZone('UTC'));
 
-        $this->assertEquals($expected, $dateTime);
+        self::assertEquals($expected, $dateTime);
 
         $dateTime = DateTimeParser::parse('20100316');
-        $this->assertEquals($expected, $dateTime);
+        self::assertEquals($expected, $dateTime);
     }
 
     /**
@@ -121,10 +121,10 @@ public function testParseICalendarDateGreaterThan4000(): void
 
         $expected = new \DateTimeImmutable('4500-12-31 00:00:00', new \DateTimeZone('UTC'));
 
-        $this->assertEquals($expected, $dateTime);
+        self::assertEquals($expected, $dateTime);
 
         $dateTime = DateTimeParser::parse('45001231');
-        $this->assertEquals($expected, $dateTime);
+        self::assertEquals($expected, $dateTime);
     }
 
     /**
@@ -136,10 +136,10 @@ public function testParseICalendarDateTimeGreaterThan4000(): void
 
         $expected = new \DateTimeImmutable('4500-12-31 23:59:59', new \DateTimeZone('UTC'));
 
-        $this->assertEquals($expected, $dateTime);
+        self::assertEquals($expected, $dateTime);
 
         $dateTime = DateTimeParser::parse('45001231T235959');
-        $this->assertEquals($expected, $dateTime);
+        self::assertEquals($expected, $dateTime);
     }
 
     /**
@@ -165,7 +165,7 @@ public function testParseICalendarDateInvalidDate(): void
      */
     public function testVCardDate(string $input, array $output): void
     {
-        $this->assertEquals(
+        self::assertEquals(
             $output,
             DateTimeParser::parseVCardDateTime($input)
         );
@@ -413,7 +413,7 @@ public function vcardDates(): array
 
     public function testDateAndOrTimeDateWithYearMonthDay(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '20150128',
             [
                 'year' => '2015',
@@ -425,7 +425,7 @@ public function testDateAndOrTimeDateWithYearMonthDay(): void
 
     public function testDateAndOrTimeDateWithYearMonth(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '2015-01',
             [
                 'year' => '2015',
@@ -436,7 +436,7 @@ public function testDateAndOrTimeDateWithYearMonth(): void
 
     public function testDateAndOrTimeDateWithMonth(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '--01',
             [
                 'month' => '01',
@@ -446,7 +446,7 @@ public function testDateAndOrTimeDateWithMonth(): void
 
     public function testDateAndOrTimeDateWithMonthDay(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '--0128',
             [
                 'month' => '01',
@@ -457,7 +457,7 @@ public function testDateAndOrTimeDateWithMonthDay(): void
 
     public function testDateAndOrTimeDateWithDay(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '---28',
             [
                 'date' => '28',
@@ -467,7 +467,7 @@ public function testDateAndOrTimeDateWithDay(): void
 
     public function testDateAndOrTimeTimeWithHour(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '13',
             [
                 'hour' => '13',
@@ -477,7 +477,7 @@ public function testDateAndOrTimeTimeWithHour(): void
 
     public function testDateAndOrTimeTimeWithHourMinute(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '1353',
             [
                 'hour' => '13',
@@ -488,7 +488,7 @@ public function testDateAndOrTimeTimeWithHourMinute(): void
 
     public function testDateAndOrTimeTimeWithHourSecond(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '135301',
             [
                 'hour' => '13',
@@ -500,7 +500,7 @@ public function testDateAndOrTimeTimeWithHourSecond(): void
 
     public function testDateAndOrTimeTimeWithMinute(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '-53',
             [
                 'minute' => '53',
@@ -510,7 +510,7 @@ public function testDateAndOrTimeTimeWithMinute(): void
 
     public function testDateAndOrTimeTimeWithMinuteSecond(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '-5301',
             [
                 'minute' => '53',
@@ -521,7 +521,7 @@ public function testDateAndOrTimeTimeWithMinuteSecond(): void
 
     public function testDateAndOrTimeTimeWithSecond(): void
     {
-        $this->assertTrue(true);
+        self::assertTrue(true);
 
         /*
          * This is unreachable due to a conflict between date and time pattern.
@@ -531,7 +531,7 @@ public function testDateAndOrTimeTimeWithSecond(): void
 
     public function testDateAndOrTimeTimeWithSecondZ(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '--01Z',
             [
                 'second' => '01',
@@ -542,7 +542,7 @@ public function testDateAndOrTimeTimeWithSecondZ(): void
 
     public function testDateAndOrTimeTimeWithSecondTZ(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '--01+1234',
             [
                 'second' => '01',
@@ -553,7 +553,7 @@ public function testDateAndOrTimeTimeWithSecondTZ(): void
 
     public function testDateAndOrTimeDateTimeWithYearMonthDayHour(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '20150128T13',
             [
                 'year' => '2015',
@@ -566,7 +566,7 @@ public function testDateAndOrTimeDateTimeWithYearMonthDayHour(): void
 
     public function testDateAndOrTimeDateTimeWithMonthDayHour(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '--0128T13',
             [
                 'month' => '01',
@@ -578,7 +578,7 @@ public function testDateAndOrTimeDateTimeWithMonthDayHour(): void
 
     public function testDateAndOrTimeDateTimeWithDayHour(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '---28T13',
             [
                 'date' => '28',
@@ -589,7 +589,7 @@ public function testDateAndOrTimeDateTimeWithDayHour(): void
 
     public function testDateAndOrTimeDateTimeWithDayHourMinute(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '---28T1353',
             [
                 'date' => '28',
@@ -601,7 +601,7 @@ public function testDateAndOrTimeDateTimeWithDayHourMinute(): void
 
     public function testDateAndOrTimeDateTimeWithDayHourMinuteSecond(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '---28T135301',
             [
                 'date' => '28',
@@ -614,7 +614,7 @@ public function testDateAndOrTimeDateTimeWithDayHourMinuteSecond(): void
 
     public function testDateAndOrTimeDateTimeWithDayHourZ(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '---28T13Z',
             [
                 'date' => '28',
@@ -626,7 +626,7 @@ public function testDateAndOrTimeDateTimeWithDayHourZ(): void
 
     public function testDateAndOrTimeDateTimeWithDayHourTZ(): void
     {
-        $this->assertDateAndOrTimeEqualsTo(
+        self::assertDateAndOrTimeEqualsTo(
             '---28T13+1234',
             [
                 'date' => '28',
@@ -638,7 +638,7 @@ public function testDateAndOrTimeDateTimeWithDayHourTZ(): void
 
     protected function assertDateAndOrTimeEqualsTo(string $date, array $parts): void
     {
-        $this->assertSame(
+        self::assertSame(
             DateTimeParser::parseVCardDateAndOrTime($date),
             array_merge(
                 [
diff --git a/tests/VObject/DocumentTest.php b/tests/VObject/DocumentTest.php
index d5b2f145..d03a8883 100644
--- a/tests/VObject/DocumentTest.php
+++ b/tests/VObject/DocumentTest.php
@@ -9,13 +9,13 @@ class DocumentTest extends TestCase
     public function testGetDocumentType(): void
     {
         $doc = new MockDocument('WHATEVER');
-        $this->assertEquals(Document::UNKNOWN, $doc->getDocumentType());
+        self::assertEquals(Document::UNKNOWN, $doc->getDocumentType());
     }
 
     public function testConstruct(): void
     {
         $doc = new MockDocument('VLIST');
-        $this->assertEquals('VLIST', $doc->name);
+        self::assertEquals('VLIST', $doc->name);
     }
 
     public function testCreateComponent(): void
@@ -24,11 +24,11 @@ public function testCreateComponent(): void
 
         $event = $vcal->createComponent('VEVENT');
 
-        $this->assertInstanceOf(Component\VEvent::class, $event);
+        self::assertInstanceOf(Component\VEvent::class, $event);
         $vcal->add($event);
 
         $prop = $vcal->createProperty('X-PROP', '1234256', ['X-PARAM' => '3']);
-        $this->assertInstanceOf(Property::class, $prop);
+        self::assertInstanceOf(Property::class, $prop);
 
         $event->add($prop);
 
@@ -38,7 +38,7 @@ public function testCreateComponent(): void
         );
 
         $out = $vcal->serialize();
-        $this->assertEquals("BEGIN:VCALENDAR\r\nBEGIN:VEVENT\r\nX-PROP;X-PARAM=3:1234256\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n", $out);
+        self::assertEquals("BEGIN:VCALENDAR\r\nBEGIN:VEVENT\r\nX-PROP;X-PARAM=3:1234256\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n", $out);
     }
 
     public function testCreate(): void
@@ -46,17 +46,17 @@ public function testCreate(): void
         $vcal = new Component\VCalendar([], false);
 
         $event = $vcal->create('VEVENT');
-        $this->assertInstanceOf(Component\VEvent::class, $event);
+        self::assertInstanceOf(Component\VEvent::class, $event);
 
         $prop = $vcal->create('CALSCALE');
-        $this->assertInstanceOf(Property\Text::class, $prop);
+        self::assertInstanceOf(Property\Text::class, $prop);
     }
 
     public function testGetClassNameForPropertyValue(): void
     {
         $vcal = new Component\VCalendar([], false);
-        $this->assertEquals(Property\Text::class, $vcal->getClassNameForPropertyValue('TEXT'));
-        $this->assertNull($vcal->getClassNameForPropertyValue('FOO'));
+        self::assertEquals(Property\Text::class, $vcal->getClassNameForPropertyValue('TEXT'));
+        self::assertNull($vcal->getClassNameForPropertyValue('FOO'));
     }
 
     /**
@@ -67,18 +67,18 @@ public function testDestroy(): void
         $vcal = new Component\VCalendar([], false);
         $event = $vcal->createComponent('VEVENT');
 
-        $this->assertInstanceOf(Component\VEvent::class, $event);
+        self::assertInstanceOf(Component\VEvent::class, $event);
         $vcal->add($event);
 
         $prop = $vcal->createProperty('X-PROP', '1234256', ['X-PARAM' => '3']);
 
         $event->add($prop);
 
-        $this->assertEquals($event, $prop->parent);
+        self::assertEquals($event, $prop->parent);
 
         $vcal->destroy();
 
-        $this->assertNull($prop->parent);
+        self::assertNull($prop->parent);
     }
 }
 
diff --git a/tests/VObject/ElementListTest.php b/tests/VObject/ElementListTest.php
index 9cf4c7fd..22b616fb 100644
--- a/tests/VObject/ElementListTest.php
+++ b/tests/VObject/ElementListTest.php
@@ -22,12 +22,12 @@ public function testIterate(): void
         $count = 0;
         foreach ($elemList as $key => $subcomponent) {
             ++$count;
-            $this->assertInstanceOf(Component::class, $subcomponent);
+            self::assertInstanceOf(Component::class, $subcomponent);
 
             if (3 === $count) {
-                $this->assertEquals(2, $key);
+                self::assertEquals(2, $key);
             }
         }
-        $this->assertEquals(3, $count);
+        self::assertEquals(3, $count);
     }
 }
diff --git a/tests/VObject/EmClientTest.php b/tests/VObject/EmClientTest.php
index 357df774..0af6fdb2 100644
--- a/tests/VObject/EmClientTest.php
+++ b/tests/VObject/EmClientTest.php
@@ -49,6 +49,6 @@ public function testParseTz(): void
 
         $vObject = Reader::read($str);
         $dt = $vObject->VEVENT->DTSTART->getDateTime();
-        $this->assertEquals(new \DateTimeImmutable('2011-10-08 19:30:00', new \DateTimeZone('America/Chicago')), $dt);
+        self::assertEquals(new \DateTimeImmutable('2011-10-08 19:30:00', new \DateTimeZone('America/Chicago')), $dt);
     }
 }
diff --git a/tests/VObject/EmptyParameterTest.php b/tests/VObject/EmptyParameterTest.php
index 86b3c82d..ab197ab3 100644
--- a/tests/VObject/EmptyParameterTest.php
+++ b/tests/VObject/EmptyParameterTest.php
@@ -20,14 +20,14 @@ public function testRead(): void
 
         $vcard = Reader::read($input);
 
-        $this->assertInstanceOf(Component\VCard::class, $vcard);
+        self::assertInstanceOf(Component\VCard::class, $vcard);
         $vcard = $vcard->convert(\Sabre\VObject\Document::VCARD30);
         $vcard = $vcard->serialize();
 
         $converted = Reader::read($vcard);
         $converted->validate();
 
-        $this->assertTrue(isset($converted->EMAIL['X-INTERN']));
+        self::assertTrue(isset($converted->EMAIL['X-INTERN']));
 
         $version = Version::VERSION;
 
@@ -43,7 +43,7 @@ public function testRead(): void
 
 VCF;
 
-        $this->assertEquals($expected, str_replace("\r", '', $vcard));
+        self::assertEquals($expected, str_replace("\r", '', $vcard));
     }
 
     public function testVCard21Parameter(): void
@@ -64,6 +64,6 @@ public function testVCard21Parameter(): void
             '',
         ];
 
-        $this->assertEquals(implode("\r\n", $expected), $result);
+        self::assertEquals(implode("\r\n", $expected), $result);
     }
 }
diff --git a/tests/VObject/EmptyValueIssueTest.php b/tests/VObject/EmptyValueIssueTest.php
index 7c90b0dc..533ac6ab 100644
--- a/tests/VObject/EmptyValueIssueTest.php
+++ b/tests/VObject/EmptyValueIssueTest.php
@@ -25,6 +25,6 @@ public function testDecodeValue(): void
         $vobj = Reader::read($input);
 
         // Before this bug was fixed, getValue() would return nothing.
-        $this->assertEquals("This is a description\nwith a linebreak and a ; , and :", $vobj->VEVENT->DESCRIPTION->getValue());
+        self::assertEquals("This is a description\nwith a linebreak and a ; , and :", $vobj->VEVENT->DESCRIPTION->getValue());
     }
 }
diff --git a/tests/VObject/FreeBusyDataTest.php b/tests/VObject/FreeBusyDataTest.php
index 8a0aad64..10b2f403 100644
--- a/tests/VObject/FreeBusyDataTest.php
+++ b/tests/VObject/FreeBusyDataTest.php
@@ -10,7 +10,7 @@ public function testGetData(): void
     {
         $fb = new FreeBusyData(100, 200);
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 [
                     'start' => 100,
@@ -32,7 +32,7 @@ public function testAddBeginning(): void
         // Overwriting the first half
         $fb->add(100, 150, 'BUSY');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 [
                     'start' => 100,
@@ -51,7 +51,7 @@ public function testAddBeginning(): void
         // Overwriting the first half again
         $fb->add(100, 150, 'BUSY-TENTATIVE');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 [
                     'start' => 100,
@@ -78,7 +78,7 @@ public function testAddEnd(): void
         // Overwriting the first half
         $fb->add(150, 200, 'BUSY');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 [
                     'start' => 100,
@@ -105,7 +105,7 @@ public function testAddMiddle(): void
         // Overwriting the first half
         $fb->add(150, 160, 'BUSY');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 [
                     'start' => 100,
@@ -137,7 +137,7 @@ public function testAddMultiple(): void
         $fb->add(110, 120, 'BUSY');
         $fb->add(130, 140, 'BUSY');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 [
                     'start' => 100,
@@ -179,7 +179,7 @@ public function testAddMultipleOverlap(): void
         $fb->add(110, 120, 'BUSY');
         $fb->add(130, 140, 'BUSY');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 [
                     'start' => 100,
@@ -212,7 +212,7 @@ public function testAddMultipleOverlap(): void
 
         $fb->add(115, 135, 'BUSY-TENTATIVE');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 [
                     'start' => 100,
@@ -254,7 +254,7 @@ public function testAddMultipleOverlapAndMerge(): void
         $fb->add(110, 120, 'BUSY');
         $fb->add(130, 140, 'BUSY');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 [
                     'start' => 100,
@@ -287,7 +287,7 @@ public function testAddMultipleOverlapAndMerge(): void
 
         $fb->add(115, 135, 'BUSY');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 [
                     'start' => 100,
diff --git a/tests/VObject/FreeBusyGeneratorTest.php b/tests/VObject/FreeBusyGeneratorTest.php
index 8167a10c..e4715f63 100644
--- a/tests/VObject/FreeBusyGeneratorTest.php
+++ b/tests/VObject/FreeBusyGeneratorTest.php
@@ -19,7 +19,7 @@ public function testGeneratorBaseObject(): void
 
         $result = $gen->getResult();
 
-        $this->assertEquals('PUBLISH', $result->METHOD->getValue());
+        self::assertEquals('PUBLISH', $result->METHOD->getValue());
     }
 
     public function testInvalidArg(): void
@@ -78,7 +78,7 @@ public function assertFreeBusyReport(string $expected, $input, \DateTimeZone $ti
 END:VCALENDAR
 ICS;
 
-        $this->assertVObjectEqualsVObject($expected, $output);
+        self::assertVObjectEqualsVObject($expected, $output);
     }
 
     public function testSimple(): void
@@ -93,7 +93,7 @@ public function testSimple(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T120000Z/20110101T130000Z',
             $blob
         );
@@ -114,7 +114,7 @@ public function testSource(): void
         fwrite($h, $blob);
         rewind($h);
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T120000Z/20110101T130000Z',
             $h
         );
@@ -136,7 +136,7 @@ public function testOpaque(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T130000Z/20110101T140000Z',
             $blob
         );
@@ -159,7 +159,7 @@ public function testTransparent(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             '',
             $blob
         );
@@ -182,7 +182,7 @@ public function testCancelled(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             '',
             $blob
         );
@@ -205,7 +205,7 @@ public function testTentative(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY;FBTYPE=BUSY-TENTATIVE:20110101T180000Z/20110101T190000Z',
             $blob
         );
@@ -227,7 +227,7 @@ public function testOutsideTimeRange(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             '',
             $blob
         );
@@ -249,7 +249,7 @@ public function testOutsideTimeRange2(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             '',
             $blob
         );
@@ -271,7 +271,7 @@ public function testDuration(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T190000Z/20110101T200000Z',
             $blob
         );
@@ -292,7 +292,7 @@ public function testAllDay(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110102T000000Z/20110103T000000Z',
             $blob
         );
@@ -313,7 +313,7 @@ public function testNoDuration(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             '',
             $blob
         );
@@ -335,7 +335,7 @@ public function testObject(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T210000Z/20110101T220000Z',
             Reader::read($blob)
         );
@@ -359,7 +359,7 @@ public function testVFreeBusy(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             "FREEBUSY:20110103T010000Z/20110103T020000Z\n".
             'FREEBUSY:20110103T030000Z/20110103T060000Z',
             $blob
@@ -380,7 +380,7 @@ public function testYearlyRecurrence(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T220000Z/20110101T230000Z',
             $blob
         );
@@ -400,7 +400,7 @@ public function testYearlyRecurrenceDuration(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T230000Z/20110102T000000Z',
             $blob
         );
@@ -419,7 +419,7 @@ public function testFloatingTime(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T120000Z/20110101T130000Z',
             $blob
         );
@@ -438,7 +438,7 @@ public function testFloatingTimeReferenceTimeZone(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T170000Z/20110101T180000Z',
             $blob,
             new \DateTimeZone('America/Toronto')
@@ -457,7 +457,7 @@ public function testAllDay2(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T110000Z/20110102T000000Z',
             $blob
         );
@@ -475,7 +475,7 @@ public function testAllDayReferenceTimeZone(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T110000Z/20110102T050000Z',
             $blob,
             new \DateTimeZone('America/Toronto')
@@ -497,7 +497,7 @@ public function testNoValidInstances(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             '',
             $blob
         );
@@ -532,7 +532,7 @@ public function testVAvailabilitySimple(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             "FREEBUSY;FBTYPE=BUSY-UNAVAILABLE:20110101T110000Z/20110101T120000Z\n".
             "FREEBUSY:20110101T120000Z/20110101T130000Z\n".
             'FREEBUSY;FBTYPE=BUSY-UNAVAILABLE:20110101T130000Z/20110103T110000Z',
@@ -571,7 +571,7 @@ public function testVAvailabilityIrrelevant(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T120000Z/20110101T130000Z',
             $blob,
             null,
@@ -610,7 +610,7 @@ public function testVAvailabilityOfficeHours(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             "FREEBUSY;FBTYPE=BUSY-TENTATIVE:20110101T110000Z/20110101T120000Z\n".
             "FREEBUSY:20110101T120000Z/20110101T130000Z\n".
             "FREEBUSY;FBTYPE=BUSY-TENTATIVE:20110101T130000Z/20110103T090000Z\n",
@@ -658,7 +658,7 @@ public function testVAvailabilityOfficeHoursVacation(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             'FREEBUSY:20110101T110000Z/20110103T110000Z',
             $blob,
             null,
@@ -706,7 +706,7 @@ public function testVAvailabilityOfficeHoursVacation2(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertFreeBusyReport(
+        self::assertFreeBusyReport(
             "FREEBUSY;FBTYPE=BUSY-TENTATIVE:20110101T110000Z/20110101T120000Z\n".
             "FREEBUSY:20110101T120000Z/20110101T130000Z\n".
             "FREEBUSY;FBTYPE=BUSY-TENTATIVE:20110101T130000Z/20110103T090000Z\n",
diff --git a/tests/VObject/GoogleColonEscapingTest.php b/tests/VObject/GoogleColonEscapingTest.php
index 16f07b5e..fa2ba8a7 100644
--- a/tests/VObject/GoogleColonEscapingTest.php
+++ b/tests/VObject/GoogleColonEscapingTest.php
@@ -26,6 +26,6 @@ public function testDecode(): void
 VCF;
 
         $vobj = Reader::read($vcard);
-        $this->assertEquals('http://www.rooftopsolutions.nl/', $vobj->URL->getValue());
+        self::assertEquals('http://www.rooftopsolutions.nl/', $vobj->URL->getValue());
     }
 }
diff --git a/tests/VObject/ICalendar/AttachParseTest.php b/tests/VObject/ICalendar/AttachParseTest.php
index b55659d5..cb54eb34 100644
--- a/tests/VObject/ICalendar/AttachParseTest.php
+++ b/tests/VObject/ICalendar/AttachParseTest.php
@@ -24,7 +24,7 @@ public function testParseAttach(): void
         $vcal = Reader::read($vcal);
         $prop = $vcal->VEVENT->ATTACH;
 
-        $this->assertInstanceOf(Uri::class, $prop);
-        $this->assertEquals('ftp://example.com/pub/reports/r-960812.ps', $prop->getValue());
+        self::assertInstanceOf(Uri::class, $prop);
+        self::assertEquals('ftp://example.com/pub/reports/r-960812.ps', $prop->getValue());
     }
 }
diff --git a/tests/VObject/ITip/BrokerTester.php b/tests/VObject/ITip/BrokerTester.php
index 8f79536c..cc01ed3f 100644
--- a/tests/VObject/ITip/BrokerTester.php
+++ b/tests/VObject/ITip/BrokerTester.php
@@ -28,19 +28,19 @@ public function parse($oldMessage, $newMessage, array $expected = [], string $cu
         $broker = new Broker();
         $result = $broker->parseEvent($newMessage, $currentUser, $oldMessage);
 
-        $this->assertSameSize($expected, $result);
+        self::assertSameSize($expected, $result);
 
         foreach ($expected as $index => $ex) {
             $message = $result[$index];
 
             foreach ($ex as $key => $val) {
                 if ('message' === $key) {
-                    $this->assertVObjectEqualsVObject(
+                    self::assertVObjectEqualsVObject(
                         $val,
                         $message->message->serialize()
                     );
                 } else {
-                    $this->assertEquals($val, $message->$key);
+                    self::assertEquals($val, $message->$key);
                 }
             }
         }
@@ -93,12 +93,12 @@ public function process($input, $existingObject = null, $expected = false): void
         $result = $broker->processMessage($message, $existingObject);
 
         if (is_null($expected)) {
-            $this->assertTrue(!$result);
+            self::assertTrue(!$result);
 
             return;
         }
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $result
         );
diff --git a/tests/VObject/ITip/BrokerTimezoneInParseEventInfoWithoutMasterTest.php b/tests/VObject/ITip/BrokerTimezoneInParseEventInfoWithoutMasterTest.php
index 8e6b3aed..e3f18f15 100644
--- a/tests/VObject/ITip/BrokerTimezoneInParseEventInfoWithoutMasterTest.php
+++ b/tests/VObject/ITip/BrokerTimezoneInParseEventInfoWithoutMasterTest.php
@@ -72,7 +72,7 @@ public function testTimezoneInParseEventInfoWithoutMaster(): void
         $reflectionMethod = new \ReflectionMethod($broker, 'parseEventInfo');
         $reflectionMethod->setAccessible(true);
         $data = $reflectionMethod->invoke($broker, $calendar);
-        $this->assertInstanceOf('DateTimeZone', $data['timezone']);
-        $this->assertEquals('Europe/Minsk', $data['timezone']->getName());
+        self::assertInstanceOf('DateTimeZone', $data['timezone']);
+        self::assertEquals('Europe/Minsk', $data['timezone']->getName());
     }
 }
diff --git a/tests/VObject/ITip/MessageTest.php b/tests/VObject/ITip/MessageTest.php
index 25bfcbdc..3196c1aa 100644
--- a/tests/VObject/ITip/MessageTest.php
+++ b/tests/VObject/ITip/MessageTest.php
@@ -9,7 +9,7 @@ class MessageTest extends TestCase
     public function testNoScheduleStatus(): void
     {
         $message = new Message();
-        $this->assertFalse($message->getScheduleStatus());
+        self::assertFalse($message->getScheduleStatus());
     }
 
     public function testScheduleStatus(): void
@@ -17,7 +17,7 @@ public function testScheduleStatus(): void
         $message = new Message();
         $message->scheduleStatus = '1.2;Delivered';
 
-        $this->assertEquals('1.2', $message->getScheduleStatus());
+        self::assertEquals('1.2', $message->getScheduleStatus());
     }
 
     public function testUnexpectedScheduleStatus(): void
@@ -25,6 +25,6 @@ public function testUnexpectedScheduleStatus(): void
         $message = new Message();
         $message->scheduleStatus = '9.9.9';
 
-        $this->assertEquals('9.9.9', $message->getScheduleStatus());
+        self::assertEquals('9.9.9', $message->getScheduleStatus());
     }
 }
diff --git a/tests/VObject/Issue153Test.php b/tests/VObject/Issue153Test.php
index 42576ed8..83e56cf5 100644
--- a/tests/VObject/Issue153Test.php
+++ b/tests/VObject/Issue153Test.php
@@ -9,6 +9,6 @@ class Issue153Test extends TestCase
     public function testRead(): void
     {
         $obj = Reader::read(file_get_contents(dirname(__FILE__).'/issue153.vcf'));
-        $this->assertEquals('Test Benutzer', (string) $obj->FN);
+        self::assertEquals('Test Benutzer', (string) $obj->FN);
     }
 }
diff --git a/tests/VObject/Issue259Test.php b/tests/VObject/Issue259Test.php
index ac6c2cdc..5c014ad3 100644
--- a/tests/VObject/Issue259Test.php
+++ b/tests/VObject/Issue259Test.php
@@ -17,7 +17,7 @@ public function testParsingJcalWithUntil(): void
         $event = reset($eventAsArray);
         $rruleAsArray = $event->select('RRULE');
         $rrule = reset($rruleAsArray);
-        $this->assertNotNull($rrule);
-        $this->assertEquals('FREQ=MONTHLY;UNTIL=20160101T220000Z', $rrule->getValue());
+        self::assertNotNull($rrule);
+        self::assertEquals('FREQ=MONTHLY;UNTIL=20160101T220000Z', $rrule->getValue());
     }
 }
diff --git a/tests/VObject/Issue36WorkAroundTest.php b/tests/VObject/Issue36WorkAroundTest.php
index bac66bbb..e6c4f254 100644
--- a/tests/VObject/Issue36WorkAroundTest.php
+++ b/tests/VObject/Issue36WorkAroundTest.php
@@ -34,6 +34,6 @@ public function testWorkaround(): void
 
         // If this does not throw an exception, it's all good.
         $it = new Recur\EventIterator($obj, '1833bd44-188b-405c-9f85-1a12105318aa');
-        $this->assertInstanceOf(Recur\EventIterator::class, $it);
+        self::assertInstanceOf(Recur\EventIterator::class, $it);
     }
 }
diff --git a/tests/VObject/Issue40Test.php b/tests/VObject/Issue40Test.php
index f44a28c2..3d280ed4 100644
--- a/tests/VObject/Issue40Test.php
+++ b/tests/VObject/Issue40Test.php
@@ -27,6 +27,6 @@ public function testEncode(): void
             '',
         ]);
 
-        $this->assertEquals($expected, $card->serialize());
+        self::assertEquals($expected, $card->serialize());
     }
 }
diff --git a/tests/VObject/Issue64Test.php b/tests/VObject/Issue64Test.php
index f102b111..110f91fa 100644
--- a/tests/VObject/Issue64Test.php
+++ b/tests/VObject/Issue64Test.php
@@ -14,6 +14,6 @@ public function testRead(): void
 
         $converted = Reader::read($vcard);
 
-        $this->assertInstanceOf(Component\VCard::class, $converted);
+        self::assertInstanceOf(Component\VCard::class, $converted);
     }
 }
diff --git a/tests/VObject/Issue96Test.php b/tests/VObject/Issue96Test.php
index 0f523ced..137149e8 100644
--- a/tests/VObject/Issue96Test.php
+++ b/tests/VObject/Issue96Test.php
@@ -18,7 +18,7 @@ public function testRead(): void
 VCF;
 
         $vcard = Reader::read($input, Reader::OPTION_FORGIVING);
-        $this->assertInstanceOf(Component\VCard::class, $vcard);
-        $this->assertEquals('http://www.example.org', $vcard->URL->getValue());
+        self::assertInstanceOf(Component\VCard::class, $vcard);
+        self::assertEquals('http://www.example.org', $vcard->URL->getValue());
     }
 }
diff --git a/tests/VObject/JCalTest.php b/tests/VObject/JCalTest.php
index 826d95bc..fa347966 100644
--- a/tests/VObject/JCalTest.php
+++ b/tests/VObject/JCalTest.php
@@ -144,6 +144,6 @@ public function testToJCal(): void
             ],
         ];
 
-        $this->assertEquals($expected, $cal->jsonSerialize());
+        self::assertEquals($expected, $cal->jsonSerialize());
     }
 }
diff --git a/tests/VObject/JCardTest.php b/tests/VObject/JCardTest.php
index 37dd7b47..cbd56ba9 100644
--- a/tests/VObject/JCardTest.php
+++ b/tests/VObject/JCardTest.php
@@ -189,6 +189,6 @@ public function testToJCard(): void
             ],
         ];
 
-        $this->assertEquals($expected, $card->jsonSerialize());
+        self::assertEquals($expected, $card->jsonSerialize());
     }
 }
diff --git a/tests/VObject/LineFoldingIssueTest.php b/tests/VObject/LineFoldingIssueTest.php
index 4b9f0190..5e636d36 100644
--- a/tests/VObject/LineFoldingIssueTest.php
+++ b/tests/VObject/LineFoldingIssueTest.php
@@ -18,6 +18,6 @@ public function testRead(): void
 ICS;
 
         $obj = Reader::read($event);
-        $this->assertEquals($event, $obj->serialize());
+        self::assertEquals($event, $obj->serialize());
     }
 }
diff --git a/tests/VObject/ParameterTest.php b/tests/VObject/ParameterTest.php
index 3c59b774..4c9f2c77 100644
--- a/tests/VObject/ParameterTest.php
+++ b/tests/VObject/ParameterTest.php
@@ -11,8 +11,8 @@ public function testSetup(): void
         $cal = new Component\VCalendar();
 
         $param = new Parameter($cal, 'name', 'value');
-        $this->assertEquals('NAME', $param->name);
-        $this->assertEquals('value', $param->getValue());
+        self::assertEquals('NAME', $param->name);
+        self::assertEquals('value', $param->getValue());
     }
 
     public function testSetupNameLess(): void
@@ -20,9 +20,9 @@ public function testSetupNameLess(): void
         $card = new Component\VCard();
 
         $param = new Parameter($card, null, 'URL');
-        $this->assertEquals('VALUE', $param->name);
-        $this->assertEquals('URL', $param->getValue());
-        $this->assertTrue($param->noName);
+        self::assertEquals('VALUE', $param->name);
+        self::assertEquals('URL', $param->getValue());
+        self::assertTrue($param->noName);
     }
 
     public function testModify(): void
@@ -31,54 +31,54 @@ public function testModify(): void
 
         $param = new Parameter($cal, 'name', null);
         $param->addValue(1);
-        $this->assertEquals([1], $param->getParts());
+        self::assertEquals([1], $param->getParts());
 
         $param->setParts([1, 2]);
-        $this->assertEquals([1, 2], $param->getParts());
+        self::assertEquals([1, 2], $param->getParts());
 
         $param->addValue(3);
-        $this->assertEquals([1, 2, 3], $param->getParts());
+        self::assertEquals([1, 2, 3], $param->getParts());
 
         $param->setValue(4);
         $param->addValue(5);
-        $this->assertEquals([4, 5], $param->getParts());
+        self::assertEquals([4, 5], $param->getParts());
     }
 
     public function testCastToString(): void
     {
         $cal = new Component\VCalendar();
         $param = new Parameter($cal, 'name', 'value');
-        $this->assertEquals('value', $param->__toString());
-        $this->assertEquals('value', (string) $param);
+        self::assertEquals('value', $param->__toString());
+        self::assertEquals('value', (string) $param);
     }
 
     public function testCastNullToString(): void
     {
         $cal = new Component\VCalendar();
         $param = new Parameter($cal, 'name', null);
-        $this->assertEquals('', $param->__toString());
-        $this->assertEquals('', (string) $param);
+        self::assertEquals('', $param->__toString());
+        self::assertEquals('', (string) $param);
     }
 
     public function testSerialize(): void
     {
         $cal = new Component\VCalendar();
         $param = new Parameter($cal, 'name', 'value');
-        $this->assertEquals('NAME=value', $param->serialize());
+        self::assertEquals('NAME=value', $param->serialize());
     }
 
     public function testSerializeEmpty(): void
     {
         $cal = new Component\VCalendar();
         $param = new Parameter($cal, 'name', null);
-        $this->assertEquals('NAME=', $param->serialize());
+        self::assertEquals('NAME=', $param->serialize());
     }
 
     public function testSerializeComplex(): void
     {
         $cal = new Component\VCalendar();
         $param = new Parameter($cal, 'name', ['val1', 'val2;', 'val3^', "val4\n", 'val5"']);
-        $this->assertEquals('NAME=val1,"val2;","val3^^","val4^n","val5^\'"', $param->serialize());
+        self::assertEquals('NAME=val1,"val2;","val3^^","val4^n","val5^\'"', $param->serialize());
     }
 
     /**
@@ -91,7 +91,7 @@ public function testSerializePlusSign(): void
     {
         $cal = new Component\VCalendar();
         $param = new Parameter($cal, 'EMAIL', 'user+something@example.org');
-        $this->assertEquals('EMAIL="user+something@example.org"', $param->serialize());
+        self::assertEquals('EMAIL="user+something@example.org"', $param->serialize());
     }
 
     public function testIterate(): void
@@ -105,20 +105,20 @@ public function testIterate(): void
             $result[] = $value;
         }
 
-        $this->assertEquals([1, 2, 3, 4], $result);
+        self::assertEquals([1, 2, 3, 4], $result);
     }
 
     public function testSerializeColon(): void
     {
         $cal = new Component\VCalendar();
         $param = new Parameter($cal, 'name', 'va:lue');
-        $this->assertEquals('NAME="va:lue"', $param->serialize());
+        self::assertEquals('NAME="va:lue"', $param->serialize());
     }
 
     public function testSerializeSemiColon(): void
     {
         $cal = new Component\VCalendar();
         $param = new Parameter($cal, 'name', 'va;lue');
-        $this->assertEquals('NAME="va;lue"', $param->serialize());
+        self::assertEquals('NAME="va;lue"', $param->serialize());
     }
 }
diff --git a/tests/VObject/Parser/JsonTest.php b/tests/VObject/Parser/JsonTest.php
index 3d118618..996e834e 100644
--- a/tests/VObject/Parser/JsonTest.php
+++ b/tests/VObject/Parser/JsonTest.php
@@ -189,9 +189,9 @@ public function testRoundTripJCard(): void
 END:VCARD
 
 VCF;
-        $this->assertEquals($expected, str_replace("\r", '', $result));
+        self::assertEquals($expected, str_replace("\r", '', $result));
 
-        $this->assertEquals(
+        self::assertEquals(
             $input,
             $vobj->jsonSerialize()
         );
@@ -345,9 +345,9 @@ public function testRoundTripJCal(): void
 END:VCALENDAR
 
 VCF;
-        $this->assertEquals($expected, str_replace("\r", '', $result));
+        self::assertEquals($expected, str_replace("\r", '', $result));
 
-        $this->assertEquals(
+        self::assertEquals(
             $input,
             $vobj->jsonSerialize()
         );
@@ -369,7 +369,7 @@ public function testParseStreamArg(): void
         rewind($stream);
 
         $result = VObject\Reader::readJson($stream, 0);
-        $this->assertEquals('foo', $result->FN->getValue());
+        self::assertEquals('foo', $result->FN->getValue());
     }
 
     public function testParseInvalidData(): void
diff --git a/tests/VObject/Parser/MimeDirTest.php b/tests/VObject/Parser/MimeDirTest.php
index af5f5181..24656f88 100644
--- a/tests/VObject/Parser/MimeDirTest.php
+++ b/tests/VObject/Parser/MimeDirTest.php
@@ -31,7 +31,7 @@ public function testDecodeLatin1(): void
         $mimeDir = new MimeDir();
         $mimeDir->setCharset('ISO-8859-1');
         $vcard = $mimeDir->parse($vcard);
-        $this->assertEquals("umlaut u - \xC3\xBC", $vcard->FN->getValue());
+        self::assertEquals("umlaut u - \xC3\xBC", $vcard->FN->getValue());
     }
 
     public function testDecodeInlineLatin1(): void
@@ -45,7 +45,7 @@ public function testDecodeInlineLatin1(): void
 
         $mimeDir = new MimeDir();
         $vcard = $mimeDir->parse($vcard);
-        $this->assertEquals("umlaut u - \xC3\xBC", $vcard->FN->getValue());
+        self::assertEquals("umlaut u - \xC3\xBC", $vcard->FN->getValue());
     }
 
     public function testIgnoreCharsetVCard30(): void
@@ -59,7 +59,7 @@ public function testIgnoreCharsetVCard30(): void
 
         $mimeDir = new MimeDir();
         $vcard = $mimeDir->parse($vcard);
-        $this->assertEquals("foo-bar - \xFC", $vcard->FN->getValue());
+        self::assertEquals("foo-bar - \xFC", $vcard->FN->getValue());
     }
 
     public function testDontDecodeLatin1(): void
@@ -77,7 +77,7 @@ public function testDontDecodeLatin1(): void
         // the encoding was set to UTF-8. The result is actually invalid
         // and the validator should report this, but it tests effectively
         // that we pass through the string byte-by-byte.
-        $this->assertEquals("umlaut u - \xFC", $vcard->FN->getValue());
+        self::assertEquals("umlaut u - \xFC", $vcard->FN->getValue());
     }
 
     public function testDecodeUnsupportedCharset(): void
@@ -113,7 +113,7 @@ public function testDecodeWindows1252(): void
         $mimeDir = new MimeDir();
         $mimeDir->setCharset('Windows-1252');
         $vcard = $mimeDir->parse($vcard);
-        $this->assertEquals("Euro \xE2\x82\xAC", $vcard->FN->getValue());
+        self::assertEquals("Euro \xE2\x82\xAC", $vcard->FN->getValue());
     }
 
     public function testDecodeWindows1252Inline(): void
@@ -127,7 +127,7 @@ public function testDecodeWindows1252Inline(): void
 
         $mimeDir = new MimeDir();
         $vcard = $mimeDir->parse($vcard);
-        $this->assertEquals("Euro \xE2\x82\xAC", $vcard->FN->getValue());
+        self::assertEquals("Euro \xE2\x82\xAC", $vcard->FN->getValue());
     }
 
     public function testCaseInsensitiveInlineCharset(): void
@@ -143,8 +143,8 @@ public function testCaseInsensitiveInlineCharset(): void
         $mimeDir = new MimeDir();
         $vcard = $mimeDir->parse($vcard);
         // we can do a simple assertion here. As long as we don't get an exception, everything is thing
-        $this->assertEquals('Euro', $vcard->FN->getValue());
-        $this->assertEquals('Test2', $vcard->N->getValue());
+        self::assertEquals('Euro', $vcard->FN->getValue());
+        self::assertEquals('Test2', $vcard->N->getValue());
     }
 
     public function testParsingTwiceSameContent(): void
@@ -164,7 +164,7 @@ public function testParsingTwiceSameContent(): void
         $mimeDir = new MimeDir();
         $vcard = $mimeDir->parse($card);
         // we can do a simple assertion here. As long as we don't get an exception, everything is fine
-        $this->assertEquals('20220612', $vcard->VEVENT->DTSTART->getValue());
+        self::assertEquals('20220612', $vcard->VEVENT->DTSTART->getValue());
     }
 
     /**
@@ -176,7 +176,7 @@ public function testBrokenMultilineContentDoesNotBreakImportWhenSetToIgnoreBroke
     {
         $mimeDir = new MimeDir(null, MimeDir::OPTION_IGNORE_INVALID_LINES);
         $vcalendar = $mimeDir->parse($vcalendar);
-        $this->assertInstanceOf(VCalendar::class, $vcalendar);
+        self::assertInstanceOf(VCalendar::class, $vcalendar);
     }
 
     /**
diff --git a/tests/VObject/Parser/QuotedPrintableTest.php b/tests/VObject/Parser/QuotedPrintableTest.php
index eaec75e8..e8153604 100644
--- a/tests/VObject/Parser/QuotedPrintableTest.php
+++ b/tests/VObject/Parser/QuotedPrintableTest.php
@@ -15,10 +15,10 @@ public function testReadQuotedPrintableSimple(): void
 
         $result = Reader::read($data);
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCARD', $result->name);
-        $this->assertCount(1, $result->children());
-        $this->assertEquals('Aachen', $this->getPropertyValue($result->LABEL));
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCARD', $result->name);
+        self::assertCount(1, $result->children());
+        self::assertEquals('Aachen', $this->getPropertyValue($result->LABEL));
     }
 
     public function testReadQuotedPrintableNewlineSoft(): void
@@ -26,10 +26,10 @@ public function testReadQuotedPrintableNewlineSoft(): void
         $data = "BEGIN:VCARD\r\nLABEL;ENCODING=QUOTED-PRINTABLE:Aa=\r\n ch=\r\n en\r\nEND:VCARD";
         $result = Reader::read($data);
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCARD', $result->name);
-        $this->assertCount(1, $result->children());
-        $this->assertEquals('Aachen', $this->getPropertyValue($result->LABEL));
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCARD', $result->name);
+        self::assertCount(1, $result->children());
+        self::assertEquals('Aachen', $this->getPropertyValue($result->LABEL));
     }
 
     public function testReadQuotedPrintableNewlineHard(): void
@@ -37,10 +37,10 @@ public function testReadQuotedPrintableNewlineHard(): void
         $data = "BEGIN:VCARD\r\nLABEL;ENCODING=QUOTED-PRINTABLE:Aachen=0D=0A=\r\n Germany\r\nEND:VCARD";
         $result = Reader::read($data);
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCARD', $result->name);
-        $this->assertCount(1, $result->children());
-        $this->assertEquals("Aachen\r\nGermany", $this->getPropertyValue($result->LABEL));
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCARD', $result->name);
+        self::assertCount(1, $result->children());
+        self::assertEquals("Aachen\r\nGermany", $this->getPropertyValue($result->LABEL));
     }
 
     public function testReadQuotedPrintableCompatibilityMS(): void
@@ -48,10 +48,10 @@ public function testReadQuotedPrintableCompatibilityMS(): void
         $data = "BEGIN:VCARD\r\nLABEL;ENCODING=QUOTED-PRINTABLE:Aachen=0D=0A=\r\nDeutschland:okay\r\nEND:VCARD";
         $result = Reader::read($data, Reader::OPTION_FORGIVING);
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCARD', $result->name);
-        $this->assertCount(1, $result->children());
-        $this->assertEquals("Aachen\r\nDeutschland:okay", $this->getPropertyValue($result->LABEL));
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCARD', $result->name);
+        self::assertCount(1, $result->children());
+        self::assertEquals("Aachen\r\nDeutschland:okay", $this->getPropertyValue($result->LABEL));
     }
 
     public function testReadQuotesPrintableCompoundValues(): void
@@ -67,7 +67,7 @@ public function testReadQuotesPrintableCompoundValues(): void
 VCF;
 
         $result = Reader::read($data, Reader::OPTION_FORGIVING);
-        $this->assertEquals([
+        self::assertEquals([
             '', '', 'Münster Str. 1', 'Münster', '', '48143', 'Deutschland',
         ], $result->ADR->getParts());
     }
diff --git a/tests/VObject/Parser/XmlTest.php b/tests/VObject/Parser/XmlTest.php
index 8e7deca9..2276f7fd 100644
--- a/tests/VObject/Parser/XmlTest.php
+++ b/tests/VObject/Parser/XmlTest.php
@@ -11,7 +11,7 @@ class XmlTest extends TestCase
 
     public function testRFC6321Example1(): void
     {
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -212,7 +212,7 @@ public function testRFC6321Example2(): void
 XML;
 
         $component = VObject\Reader::readXML($xml);
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             'BEGIN:VCALENDAR'."\n".
             'VERSION:2.0'."\n".
             'PRODID:-//Example Inc.//Example Client//EN'."\n".
@@ -264,7 +264,7 @@ public function testRFC6321Example2(): void
      */
     public function testRFC6321Section3Part2(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -282,7 +282,7 @@ public function testRFC6321Section3Part2(): void
      */
     public function testRFC6321Section3Part3(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -327,7 +327,7 @@ public function testRFC6321Section3Part3(): void
      */
     public function testRFC6321Section3Part4Part1Part2(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -354,7 +354,7 @@ public function testRFC6321Section3Part4Part1Part2(): void
     public function testRFC6321Section3Part4Part1Part3(): void
     {
         // Example 1 of RFC5545, Section 3.8.8.3.
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -375,7 +375,7 @@ public function testRFC6321Section3Part4Part1Part3(): void
         );
 
         // Example 2 of RFC5545, Section 3.8.8.3.
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -397,7 +397,7 @@ public function testRFC6321Section3Part4Part1Part3(): void
         );
 
         // Example 3 of RFC5545, Section 3.8.8.3.
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -420,7 +420,7 @@ public function testRFC6321Section3Part4Part1Part3(): void
         );
 
         // Example 4 of RFC5545, Section 3.8.8.3.
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -441,7 +441,7 @@ public function testRFC6321Section3Part4Part1Part3(): void
         );
 
         // Example 5 of RFC5545, Section 3.8.8.3.
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -468,7 +468,7 @@ public function testRFC6321Section3Part4Part1Part3(): void
      */
     public function testRFC6321Section3Part6Part1(): void
     {
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -488,7 +488,7 @@ public function testRFC6321Section3Part6Part1(): void
         );
 
         // In vCard 4, BINARY no longer exists and is replaced by URI.
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -513,7 +513,7 @@ public function testRFC6321Section3Part6Part1(): void
      */
     public function testRFC6321Section3Part6Part2(): void
     {
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -541,7 +541,7 @@ public function testRFC6321Section3Part6Part2(): void
      */
     public function testRFC6321Section3Part6Part3(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -566,7 +566,7 @@ public function testRFC6321Section3Part6Part3(): void
      */
     public function testRFC6321Section3Part6Part4(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -591,7 +591,7 @@ public function testRFC6321Section3Part6Part4(): void
      */
     public function testRFC6321Section3Part6Part5(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -616,7 +616,7 @@ public function testRFC6321Section3Part6Part5(): void
      */
     public function testRFC6321Section3Part6Part6(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -650,7 +650,7 @@ public function testRFC6321Section3Part6Part7(): void
      */
     public function testRFC6321Section3Part6Part8(): void
     {
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -669,7 +669,7 @@ public function testRFC6321Section3Part6Part8(): void
             'END:VCALENDAR'."\n"
         );
 
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -694,7 +694,7 @@ public function testRFC6321Section3Part6Part8(): void
      */
     public function testRFC6321Section3Part6Part9(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -716,7 +716,7 @@ public function testRFC6321Section3Part6Part9(): void
             'END:VCALENDAR'."\n"
         );
 
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -744,7 +744,7 @@ public function testRFC6321Section3Part6Part9(): void
      */
     public function testRFC6321Section3Part6Part10(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -774,7 +774,7 @@ public function testRFC6321Section3Part6Part10(): void
      */
     public function testRFC6321Section3Part6Part11(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -799,7 +799,7 @@ public function testRFC6321Section3Part6Part11(): void
      */
     public function testRFC6321Section3Part6Part12(): void
     {
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -824,7 +824,7 @@ public function testRFC6321Section3Part6Part12(): void
      */
     public function testRFC6321Section3Part6Part13(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -850,7 +850,7 @@ public function testRFC6321Section3Part6Part13(): void
     public function testRFC6321Section3Part6Part14(): void
     {
         // Example 1 of RFC5545, Section 3.3.14.
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -870,7 +870,7 @@ public function testRFC6321Section3Part6Part14(): void
         );
 
         // Example 2 of RFC5545, Section 3.3.14.
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -895,7 +895,7 @@ public function testRFC6321Section3Part6Part14(): void
      */
     public function testRFC6321Section5(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -914,7 +914,7 @@ public function testRFC6321Section5(): void
             'END:VCALENDAR'."\n"
         );
 
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -941,7 +941,7 @@ public function testRFC6321Section5(): void
 
     public function testRDateWithDateTime(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -960,7 +960,7 @@ public function testRDateWithDateTime(): void
             'END:VCALENDAR'."\n"
         );
 
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -983,7 +983,7 @@ public function testRDateWithDateTime(): void
 
     public function testRDateWithDate(): void
     {
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -1002,7 +1002,7 @@ public function testRDateWithDate(): void
             'END:VCALENDAR'."\n"
         );
 
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -1026,7 +1026,7 @@ public function testRDateWithDate(): void
 
     public function testRDateWithPeriod(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -1053,7 +1053,7 @@ public function testRDateWithPeriod(): void
             'END:VCALENDAR'."\n"
         );
 
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">
@@ -1091,7 +1091,7 @@ public function testRDateWithPeriod(): void
      */
     public function testRFC6351Basic(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1123,7 +1123,7 @@ public function testRFC6351Basic(): void
      */
     public function testRFC6351Example1(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1167,7 +1167,7 @@ public function testRFC6351Example1(): void
      */
     public function testRFC6351Section5(): void
     {
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1191,7 +1191,7 @@ public function testRFC6351Section5(): void
             'END:VCARD'."\n"
         );
 
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1221,7 +1221,7 @@ public function testRFC6351Section5(): void
      */
     public function testRFC6351Section5Group(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1261,7 +1261,7 @@ public function testRFC6351Section5Group(): void
      */
     public function testRFC6351Section5Part1NoNamespace(): void
     {
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1290,7 +1290,7 @@ public function testRFC6351Section5Part1NoNamespace(): void
      */
     public function testRFC6351ValueDateWithYearMonthDay(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1314,7 +1314,7 @@ public function testRFC6351ValueDateWithYearMonthDay(): void
      */
     public function testRFC6351ValueDateWithYearMonth(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1338,7 +1338,7 @@ public function testRFC6351ValueDateWithYearMonth(): void
      */
     public function testRFC6351ValueDateWithMonth(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1362,7 +1362,7 @@ public function testRFC6351ValueDateWithMonth(): void
      */
     public function testRFC6351ValueDateWithMonthDay(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1386,7 +1386,7 @@ public function testRFC6351ValueDateWithMonthDay(): void
      */
     public function testRFC6351ValueDateWithDay(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1410,7 +1410,7 @@ public function testRFC6351ValueDateWithDay(): void
      */
     public function testRFC6351ValueTimeWithHour(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1434,7 +1434,7 @@ public function testRFC6351ValueTimeWithHour(): void
      */
     public function testRFC6351ValueTimeWithHourMinute(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1458,7 +1458,7 @@ public function testRFC6351ValueTimeWithHourMinute(): void
      */
     public function testRFC6351ValueTimeWithHourMinuteSecond(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1482,7 +1482,7 @@ public function testRFC6351ValueTimeWithHourMinuteSecond(): void
      */
     public function testRFC6351ValueTimeWithMinute(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1506,7 +1506,7 @@ public function testRFC6351ValueTimeWithMinute(): void
      */
     public function testRFC6351ValueTimeWithMinuteSecond(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1530,7 +1530,7 @@ public function testRFC6351ValueTimeWithMinuteSecond(): void
      */
     public function testRFC6351ValueTimeWithSecond(): void
     {
-        $this->assertTrue(true);
+        self::assertTrue(true);
 
         /*
          * According to the Relax NG Schema, there is a conflict between
@@ -1538,7 +1538,7 @@ public function testRFC6351ValueTimeWithSecond(): void
          * value-date because of the higher priority set in
          * value-date-and-or-time. So we basically skip this test.
          *
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
 <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1563,7 +1563,7 @@ public function testRFC6351ValueTimeWithSecond(): void
      */
     public function testRFC6351ValueTimeWithSecondZ(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1587,7 +1587,7 @@ public function testRFC6351ValueTimeWithSecondZ(): void
      */
     public function testRFC6351ValueTimeWithSecondTZ(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1611,7 +1611,7 @@ public function testRFC6351ValueTimeWithSecondTZ(): void
      */
     public function testRFC6351ValueDateTimeWithYearMonthDayHour(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1635,7 +1635,7 @@ public function testRFC6351ValueDateTimeWithYearMonthDayHour(): void
      */
     public function testRFC6351ValueDateTimeWithMonthDayHour(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1659,7 +1659,7 @@ public function testRFC6351ValueDateTimeWithMonthDayHour(): void
      */
     public function testRFC6351ValueDateTimeWithDayHour(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1683,7 +1683,7 @@ public function testRFC6351ValueDateTimeWithDayHour(): void
      */
     public function testRFC6351ValueDateTimeWithDayHourMinute(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1707,7 +1707,7 @@ public function testRFC6351ValueDateTimeWithDayHourMinute(): void
      */
     public function testRFC6351ValueDateTimeWithDayHourMinuteSecond(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1731,7 +1731,7 @@ public function testRFC6351ValueDateTimeWithDayHourMinuteSecond(): void
      */
     public function testRFC6351ValueDateTimeWithDayHourZ(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1755,7 +1755,7 @@ public function testRFC6351ValueDateTimeWithDayHourZ(): void
      */
     public function testRFC6351ValueDateTimeWithDayHourTZ(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1779,7 +1779,7 @@ public function testRFC6351ValueDateTimeWithDayHourTZ(): void
      */
     public function testRFC6350Section6Part1Part3(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1803,7 +1803,7 @@ public function testRFC6350Section6Part1Part3(): void
      */
     public function testRFC6350Section6Part1Part4(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1827,7 +1827,7 @@ public function testRFC6350Section6Part1Part4(): void
      */
     public function testRFC6350Section6Part2Part1(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1851,7 +1851,7 @@ public function testRFC6350Section6Part2Part1(): void
      */
     public function testRFC6350Section6Part2Part2(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1879,7 +1879,7 @@ public function testRFC6350Section6Part2Part2(): void
      */
     public function testRFC6350Section6Part2Part3(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1904,7 +1904,7 @@ public function testRFC6350Section6Part2Part3(): void
      */
     public function testRFC6350Section6Part2Part4(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1925,7 +1925,7 @@ public function testRFC6350Section6Part2Part4(): void
 
     public function testRFC6350Section6Part2Part5(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1946,7 +1946,7 @@ public function testRFC6350Section6Part2Part5(): void
 
     public function testRFC6350Section6Part2Part6(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1970,7 +1970,7 @@ public function testRFC6350Section6Part2Part6(): void
      */
     public function testRFC6350Section6Part2Part7(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -1995,7 +1995,7 @@ public function testRFC6350Section6Part2Part7(): void
      */
     public function testRFC6350Section6Part3Part1(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2035,7 +2035,7 @@ public function testRFC6350Section6Part4Part1(): void
          * So first, we test xCard/URI to vCard/URI.
          * Then, we test xCard/TEXT to vCard/TEXT to xCard/TEXT.
          */
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2058,7 +2058,7 @@ public function testRFC6350Section6Part4Part1(): void
             'END:VCARD'."\n"
         );
 
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2087,7 +2087,7 @@ public function testRFC6350Section6Part4Part1(): void
      */
     public function testRFC6350Section6Part4Part2(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2116,7 +2116,7 @@ public function testRFC6350Section6Part4Part2(): void
      */
     public function testRFC6350Section6Part4Part3(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2145,7 +2145,7 @@ public function testRFC6350Section6Part4Part3(): void
      */
     public function testRFC6350Section6Part4Part4(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2177,7 +2177,7 @@ public function testRFC6350Section6Part4Part4(): void
      */
     public function testRFC6350Section6Part5Part1(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2201,7 +2201,7 @@ public function testRFC6350Section6Part5Part1(): void
      */
     public function testRFC6350Section6Part5Part2(): void
     {
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2219,7 +2219,7 @@ public function testRFC6350Section6Part5Part2(): void
             'END:VCARD'."\n"
         );
 
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2243,7 +2243,7 @@ public function testRFC6350Section6Part5Part2(): void
      */
     public function testRFC6350Section6Part6Part1(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2267,7 +2267,7 @@ public function testRFC6350Section6Part6Part1(): void
      */
     public function testRFC6350Section6Part6Part2(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2291,7 +2291,7 @@ public function testRFC6350Section6Part6Part2(): void
      */
     public function testRFC6350Section6Part6Part3(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2315,7 +2315,7 @@ public function testRFC6350Section6Part6Part3(): void
      */
     public function testRFC6350Section6Part6Part4(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2341,7 +2341,7 @@ public function testRFC6350Section6Part6Part4(): void
      */
     public function testRFC6350Section6Part6Part5(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2359,7 +2359,7 @@ public function testRFC6350Section6Part6Part5(): void
             'END:VCARD'."\n"
         );
 
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2395,7 +2395,7 @@ public function testRFC6350Section6Part6Part5(): void
      */
     public function testRFC6350Section6Part6Part6(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2424,7 +2424,7 @@ public function testRFC6350Section6Part6Part6(): void
      */
     public function testRFC6350Section6Part7Part1(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2451,7 +2451,7 @@ public function testRFC6350Section6Part7Part1(): void
      */
     public function testRFC6350Section6Part7Part2(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2475,7 +2475,7 @@ public function testRFC6350Section6Part7Part2(): void
      */
     public function testRFC6350Section6Part7Part3(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2496,7 +2496,7 @@ public function testRFC6350Section6Part7Part3(): void
 
     public function testRFC6350Section6Part7Part4(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2520,7 +2520,7 @@ public function testRFC6350Section6Part7Part4(): void
      */
     public function testRFC6350Section6Part7Part5(): void
     {
-        $this->assertXMLEqualsToMimeDir(
+        self::assertXMLEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2538,7 +2538,7 @@ public function testRFC6350Section6Part7Part5(): void
             'END:VCARD'."\n"
         );
 
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2562,7 +2562,7 @@ public function testRFC6350Section6Part7Part5(): void
      */
     public function testRFC6350Section6Part7Part6(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2586,7 +2586,7 @@ public function testRFC6350Section6Part7Part6(): void
      */
     public function testRFC6350Section6Part7Part7(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2611,7 +2611,7 @@ public function testRFC6350Section6Part7Part7(): void
      */
     public function testRFC6350Section6Part7Part8(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2635,7 +2635,7 @@ public function testRFC6350Section6Part7Part8(): void
      */
     public function testRFC6350Section6Part7Part9(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2654,7 +2654,7 @@ public function testRFC6350Section6Part7Part9(): void
      */
     public function testRFC6350Section6Part8Part1(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2683,7 +2683,7 @@ public function testRFC6350Section6Part8Part1(): void
      */
     public function testRFC6350Section6Part9Part1(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2712,7 +2712,7 @@ public function testRFC6350Section6Part9Part1(): void
      */
     public function testRFC6350Section6Part9Part2(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2736,7 +2736,7 @@ public function testRFC6350Section6Part9Part2(): void
      */
     public function testRFC6350Section6Part9Part3(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2765,7 +2765,7 @@ public function testRFC6350Section6Part9Part3(): void
      */
     public function testRFC6350SectionAPart3(): void
     {
-        $this->assertXMLReflexivelyEqualsToMimeDir(
+        self::assertXMLReflexivelyEqualsToMimeDir(
             <<<XML
 <?xml version="1.0" encoding="UTF-8"?>
 <vcards xmlns="urn:ietf:params:xml:ns:vcard-4.0">
@@ -2791,7 +2791,7 @@ public function testRFC6350SectionAPart3(): void
     protected function assertXMLEqualsToMimeDir(string $xml, string $mimedir): void
     {
         $component = VObject\Reader::readXML($xml);
-        $this->assertVObjectEqualsVObject($mimedir, $component);
+        self::assertVObjectEqualsVObject($mimedir, $component);
     }
 
     /**
@@ -2800,9 +2800,9 @@ protected function assertXMLEqualsToMimeDir(string $xml, string $mimedir): void
      */
     protected function assertXMLReflexivelyEqualsToMimeDir(string $xml, string $mimedir): void
     {
-        $this->assertXMLEqualsToMimeDir($xml, $mimedir);
+        self::assertXMLEqualsToMimeDir($xml, $mimedir);
 
         $component = VObject\Reader::read($mimedir);
-        $this->assertXmlStringEqualsXmlString($xml, VObject\Writer::writeXML($component));
+        self::assertXmlStringEqualsXmlString($xml, VObject\Writer::writeXML($component));
     }
 }
diff --git a/tests/VObject/Property/BooleanTest.php b/tests/VObject/Property/BooleanTest.php
index ea874f17..82684dd7 100644
--- a/tests/VObject/Property/BooleanTest.php
+++ b/tests/VObject/Property/BooleanTest.php
@@ -15,10 +15,10 @@ public function testMimeDir(): void
         $input = "BEGIN:VCARD\r\nX-AWESOME;VALUE=BOOLEAN:TRUE\r\nX-SUCKS;VALUE=BOOLEAN:FALSE\r\nEND:VCARD\r\n";
 
         $vcard = VObject\Reader::read($input);
-        $this->assertTrue($vcard->{'X-AWESOME'}->getValue());
-        $this->assertFalse($vcard->{'X-SUCKS'}->getValue());
+        self::assertTrue($vcard->{'X-AWESOME'}->getValue());
+        self::assertFalse($vcard->{'X-SUCKS'}->getValue());
 
-        $this->assertEquals('BOOLEAN', $vcard->{'X-AWESOME'}->getValueType());
-        $this->assertEquals($input, $vcard->serialize());
+        self::assertEquals('BOOLEAN', $vcard->{'X-AWESOME'}->getValueType());
+        self::assertEquals($input, $vcard->serialize());
     }
 }
diff --git a/tests/VObject/Property/CompoundTest.php b/tests/VObject/Property/CompoundTest.php
index c10ff033..073d9627 100644
--- a/tests/VObject/Property/CompoundTest.php
+++ b/tests/VObject/Property/CompoundTest.php
@@ -19,10 +19,10 @@ public function testSetParts(): void
         $elem = $vcard->createProperty('ORG');
         $elem->setParts($arr);
 
-        $this->assertEquals('ABC\, Inc.;North American Division;Marketing\;Sales', $elem->getValue());
-        $this->assertCount(3, $elem->getParts());
+        self::assertEquals('ABC\, Inc.;North American Division;Marketing\;Sales', $elem->getValue());
+        self::assertCount(3, $elem->getParts());
         $parts = $elem->getParts();
-        $this->assertEquals('Marketing;Sales', $parts[2]);
+        self::assertEquals('Marketing;Sales', $parts[2]);
     }
 
     public function testGetParts(): void
@@ -33,9 +33,9 @@ public function testGetParts(): void
         $elem = $vcard->createProperty('ORG');
         $elem->setRawMimeDirValue($str);
 
-        $this->assertCount(3, $elem->getParts());
+        self::assertCount(3, $elem->getParts());
         $parts = $elem->getParts();
-        $this->assertEquals('Marketing;Sales', $parts[2]);
+        self::assertEquals('Marketing;Sales', $parts[2]);
     }
 
     public function testGetPartsNull(): void
@@ -43,6 +43,6 @@ public function testGetPartsNull(): void
         $vcard = new VCard();
         $elem = $vcard->createProperty('ORG', null);
 
-        $this->assertCount(0, $elem->getParts());
+        self::assertCount(0, $elem->getParts());
     }
 }
diff --git a/tests/VObject/Property/FloatTest.php b/tests/VObject/Property/FloatTest.php
index a71428a4..b5fee5d7 100644
--- a/tests/VObject/Property/FloatTest.php
+++ b/tests/VObject/Property/FloatTest.php
@@ -14,14 +14,14 @@ public function testMimeDir(): void
 
         $result = $mimeDir->parse($input);
 
-        $this->assertInstanceOf(FloatValue::class, $result->{'X-FLOAT'});
+        self::assertInstanceOf(FloatValue::class, $result->{'X-FLOAT'});
 
-        $this->assertEquals([
+        self::assertEquals([
             0.234,
             1.245,
         ], $result->{'X-FLOAT'}->getParts());
 
-        $this->assertEquals(
+        self::assertEquals(
             $input,
             $result->serialize()
         );
diff --git a/tests/VObject/Property/ICalendar/CalAddressTest.php b/tests/VObject/Property/ICalendar/CalAddressTest.php
index 7c9dde6d..b774f4fb 100644
--- a/tests/VObject/Property/ICalendar/CalAddressTest.php
+++ b/tests/VObject/Property/ICalendar/CalAddressTest.php
@@ -14,7 +14,7 @@ public function testGetNormalizedValue(string $expected, string $input): void
         $vobj = new \Sabre\VObject\Component\VCalendar();
         $property = $vobj->add('ATTENDEE', $input);
 
-        $this->assertEquals(
+        self::assertEquals(
             $expected,
             $property->getNormalizedValue()
         );
diff --git a/tests/VObject/Property/ICalendar/DateTimeTest.php b/tests/VObject/Property/ICalendar/DateTimeTest.php
index c926af80..00c8be1b 100644
--- a/tests/VObject/Property/ICalendar/DateTimeTest.php
+++ b/tests/VObject/Property/ICalendar/DateTimeTest.php
@@ -24,11 +24,11 @@ public function testSetDateTime(): void
         $elem = $this->vcal->createProperty('DTSTART');
         $elem->setDateTime($dt);
 
-        $this->assertEquals('19850704T013000', (string) $elem);
-        $this->assertEquals('Europe/Amsterdam', (string) $elem['TZID']);
-        $this->assertNull($elem['VALUE']);
+        self::assertEquals('19850704T013000', (string) $elem);
+        self::assertEquals('Europe/Amsterdam', (string) $elem['TZID']);
+        self::assertNull($elem['VALUE']);
 
-        $this->assertTrue($elem->hasTime());
+        self::assertTrue($elem->hasTime());
     }
 
     public function testSetDateTimeLOCAL(): void
@@ -40,10 +40,10 @@ public function testSetDateTimeLOCAL(): void
         $elem = $this->vcal->createProperty('DTSTART');
         $elem->setDateTime($dt, true);
 
-        $this->assertEquals('19850704T013000', (string) $elem);
-        $this->assertNull($elem['TZID']);
+        self::assertEquals('19850704T013000', (string) $elem);
+        self::assertNull($elem['TZID']);
 
-        $this->assertTrue($elem->hasTime());
+        self::assertTrue($elem->hasTime());
     }
 
     public function testSetDateTimeUTC(): void
@@ -55,10 +55,10 @@ public function testSetDateTimeUTC(): void
         $elem = $this->vcal->createProperty('DTSTART');
         $elem->setDateTime($dt);
 
-        $this->assertEquals('19850704T013000Z', (string) $elem);
-        $this->assertNull($elem['TZID']);
+        self::assertEquals('19850704T013000Z', (string) $elem);
+        self::assertNull($elem['TZID']);
 
-        $this->assertTrue($elem->hasTime());
+        self::assertTrue($elem->hasTime());
     }
 
     public function testSetDateTimeFromUnixTimestamp(): void
@@ -69,10 +69,10 @@ public function testSetDateTimeFromUnixTimestamp(): void
         $elem = $this->vcal->createProperty('DTSTART');
         $elem->setDateTime($dt);
 
-        $this->assertEquals('19850704T013000Z', (string) $elem);
-        $this->assertNull($elem['TZID']);
+        self::assertEquals('19850704T013000Z', (string) $elem);
+        self::assertNull($elem['TZID']);
 
-        $this->assertTrue($elem->hasTime());
+        self::assertTrue($elem->hasTime());
     }
 
     public function testSetDateTimeLOCALTZ(): void
@@ -84,10 +84,10 @@ public function testSetDateTimeLOCALTZ(): void
         $elem = $this->vcal->createProperty('DTSTART');
         $elem->setDateTime($dt);
 
-        $this->assertEquals('19850704T013000', (string) $elem);
-        $this->assertEquals('Europe/Amsterdam', (string) $elem['TZID']);
+        self::assertEquals('19850704T013000', (string) $elem);
+        self::assertEquals('Europe/Amsterdam', (string) $elem['TZID']);
 
-        $this->assertTrue($elem->hasTime());
+        self::assertTrue($elem->hasTime());
     }
 
     public function testSetDateTimeDATE(): void
@@ -100,11 +100,11 @@ public function testSetDateTimeDATE(): void
         $elem['VALUE'] = 'DATE';
         $elem->setDateTime($dt);
 
-        $this->assertEquals('19850704', (string) $elem);
-        $this->assertNull($elem['TZID']);
-        $this->assertEquals('DATE', (string) $elem['VALUE']);
+        self::assertEquals('19850704', (string) $elem);
+        self::assertNull($elem['TZID']);
+        self::assertEquals('DATE', (string) $elem['VALUE']);
 
-        $this->assertFalse($elem->hasTime());
+        self::assertFalse($elem->hasTime());
     }
 
     public function testSetValue(): void
@@ -116,11 +116,11 @@ public function testSetValue(): void
         $elem = $this->vcal->createProperty('DTSTART');
         $elem->setValue($dt);
 
-        $this->assertEquals('19850704T013000', (string) $elem);
-        $this->assertEquals('Europe/Amsterdam', (string) $elem['TZID']);
-        $this->assertNull($elem['VALUE']);
+        self::assertEquals('19850704T013000', (string) $elem);
+        self::assertEquals('Europe/Amsterdam', (string) $elem['TZID']);
+        self::assertNull($elem['VALUE']);
 
-        $this->assertTrue($elem->hasTime());
+        self::assertTrue($elem->hasTime());
     }
 
     public function testSetValueArray(): void
@@ -134,11 +134,11 @@ public function testSetValueArray(): void
         $elem = $this->vcal->createProperty('DTSTART');
         $elem->setValue([$dt1, $dt2]);
 
-        $this->assertEquals('19850704T013000,19850704T023000', (string) $elem);
-        $this->assertEquals('Europe/Amsterdam', (string) $elem['TZID']);
-        $this->assertNull($elem['VALUE']);
+        self::assertEquals('19850704T013000,19850704T023000', (string) $elem);
+        self::assertEquals('Europe/Amsterdam', (string) $elem['TZID']);
+        self::assertNull($elem['VALUE']);
 
-        $this->assertTrue($elem->hasTime());
+        self::assertTrue($elem->hasTime());
     }
 
     public function testSetParts(): void
@@ -152,11 +152,11 @@ public function testSetParts(): void
         $elem = $this->vcal->createProperty('DTSTART');
         $elem->setParts([$dt1, $dt2]);
 
-        $this->assertEquals('19850704T013000,19850704T023000', (string) $elem);
-        $this->assertEquals('Europe/Amsterdam', (string) $elem['TZID']);
-        $this->assertNull($elem['VALUE']);
+        self::assertEquals('19850704T013000,19850704T023000', (string) $elem);
+        self::assertEquals('Europe/Amsterdam', (string) $elem['TZID']);
+        self::assertNull($elem['VALUE']);
 
-        $this->assertTrue($elem->hasTime());
+        self::assertTrue($elem->hasTime());
     }
 
     public function testSetPartsStrings(): void
@@ -167,10 +167,10 @@ public function testSetPartsStrings(): void
         $elem = $this->vcal->createProperty('DTSTART');
         $elem->setParts([$dt1, $dt2]);
 
-        $this->assertEquals('19850704T013000Z,19850704T023000Z', (string) $elem);
-        $this->assertNull($elem['VALUE']);
+        self::assertEquals('19850704T013000Z,19850704T023000Z', (string) $elem);
+        self::assertNull($elem['VALUE']);
 
-        $this->assertTrue($elem->hasTime());
+        self::assertTrue($elem->hasTime());
     }
 
     public function testGetDateTimeCached(): void
@@ -182,7 +182,7 @@ public function testGetDateTimeCached(): void
         $elem = $this->vcal->createProperty('DTSTART');
         $elem->setDateTime($dt);
 
-        $this->assertEquals($elem->getDateTime(), $dt);
+        self::assertEquals($elem->getDateTime(), $dt);
     }
 
     public function testGetDateTimeDateNULL(): void
@@ -190,7 +190,7 @@ public function testGetDateTimeDateNULL(): void
         $elem = $this->vcal->createProperty('DTSTART');
         $dt = $elem->getDateTime();
 
-        $this->assertNull($dt);
+        self::assertNull($dt);
     }
 
     public function testGetDateTimeDateDATE(): void
@@ -198,8 +198,8 @@ public function testGetDateTimeDateDATE(): void
         $elem = $this->vcal->createProperty('DTSTART', '19850704');
         $dt = $elem->getDateTime();
 
-        $this->assertInstanceOf('DateTimeImmutable', $dt);
-        $this->assertEquals('1985-07-04 00:00:00', $dt->format('Y-m-d H:i:s'));
+        self::assertInstanceOf('DateTimeImmutable', $dt);
+        self::assertEquals('1985-07-04 00:00:00', $dt->format('Y-m-d H:i:s'));
     }
 
     public function testGetDateTimeDateDATEReferenceTimeZone(): void
@@ -210,8 +210,8 @@ public function testGetDateTimeDateDATEReferenceTimeZone(): void
         $dt = $elem->getDateTime($tz);
         $dt = $dt->setTimeZone(new \DateTimeZone('UTC'));
 
-        $this->assertInstanceOf('DateTimeImmutable', $dt);
-        $this->assertEquals('1985-07-04 04:00:00', $dt->format('Y-m-d H:i:s'));
+        self::assertInstanceOf('DateTimeImmutable', $dt);
+        self::assertEquals('1985-07-04 04:00:00', $dt->format('Y-m-d H:i:s'));
     }
 
     public function testGetDateTimeDateFloating(): void
@@ -219,8 +219,8 @@ public function testGetDateTimeDateFloating(): void
         $elem = $this->vcal->createProperty('DTSTART', '19850704T013000');
         $dt = $elem->getDateTime();
 
-        $this->assertInstanceOf('DateTimeImmutable', $dt);
-        $this->assertEquals('1985-07-04 01:30:00', $dt->format('Y-m-d H:i:s'));
+        self::assertInstanceOf('DateTimeImmutable', $dt);
+        self::assertEquals('1985-07-04 01:30:00', $dt->format('Y-m-d H:i:s'));
     }
 
     public function testGetDateTimeDateFloatingReferenceTimeZone(): void
@@ -231,8 +231,8 @@ public function testGetDateTimeDateFloatingReferenceTimeZone(): void
         $dt = $elem->getDateTime($tz);
         $dt = $dt->setTimeZone(new \DateTimeZone('UTC'));
 
-        $this->assertInstanceOf('DateTimeInterface', $dt);
-        $this->assertEquals('1985-07-04 05:30:00', $dt->format('Y-m-d H:i:s'));
+        self::assertInstanceOf('DateTimeInterface', $dt);
+        self::assertEquals('1985-07-04 05:30:00', $dt->format('Y-m-d H:i:s'));
     }
 
     public function testGetDateTimeDateUTC(): void
@@ -240,9 +240,9 @@ public function testGetDateTimeDateUTC(): void
         $elem = $this->vcal->createProperty('DTSTART', '19850704T013000Z');
         $dt = $elem->getDateTime();
 
-        $this->assertInstanceOf('DateTimeImmutable', $dt);
-        $this->assertEquals('1985-07-04 01:30:00', $dt->format('Y-m-d H:i:s'));
-        $this->assertEquals('UTC', $dt->getTimeZone()->getName());
+        self::assertInstanceOf('DateTimeImmutable', $dt);
+        self::assertEquals('1985-07-04 01:30:00', $dt->format('Y-m-d H:i:s'));
+        self::assertEquals('UTC', $dt->getTimeZone()->getName());
     }
 
     public function testGetDateTimeDateLOCALTZ(): void
@@ -252,9 +252,9 @@ public function testGetDateTimeDateLOCALTZ(): void
 
         $dt = $elem->getDateTime();
 
-        $this->assertInstanceOf('DateTimeImmutable', $dt);
-        $this->assertEquals('1985-07-04 01:30:00', $dt->format('Y-m-d H:i:s'));
-        $this->assertEquals('Europe/Amsterdam', $dt->getTimeZone()->getName());
+        self::assertInstanceOf('DateTimeImmutable', $dt);
+        self::assertEquals('1985-07-04 01:30:00', $dt->format('Y-m-d H:i:s'));
+        self::assertEquals('Europe/Amsterdam', $dt->getTimeZone()->getName());
     }
 
     public function testGetDateTimeDateInvalid(): void
@@ -281,9 +281,9 @@ public function testGetDateTimeWeirdTZ(): void
 
         $dt = $elem->getDateTime();
 
-        $this->assertInstanceOf('DateTimeImmutable', $dt);
-        $this->assertEquals('1985-07-04 01:30:00', $dt->format('Y-m-d H:i:s'));
-        $this->assertEquals('Europe/Amsterdam', $dt->getTimeZone()->getName());
+        self::assertInstanceOf('DateTimeImmutable', $dt);
+        self::assertEquals('1985-07-04 01:30:00', $dt->format('Y-m-d H:i:s'));
+        self::assertEquals('Europe/Amsterdam', $dt->getTimeZone()->getName());
     }
 
     public function testGetDateTimeBadTimeZone(): void
@@ -306,9 +306,9 @@ public function testGetDateTimeBadTimeZone(): void
 
         $dt = $elem->getDateTime();
 
-        $this->assertInstanceOf('DateTimeImmutable', $dt);
-        $this->assertEquals('1985-07-04 01:30:00', $dt->format('Y-m-d H:i:s'));
-        $this->assertEquals('Canada/Eastern', $dt->getTimeZone()->getName());
+        self::assertInstanceOf('DateTimeImmutable', $dt);
+        self::assertEquals('1985-07-04 01:30:00', $dt->format('Y-m-d H:i:s'));
+        self::assertEquals('Canada/Eastern', $dt->getTimeZone()->getName());
         date_default_timezone_set($default);
     }
 
@@ -317,15 +317,15 @@ public function testUpdateValueParameter(): void
         $dtStart = $this->vcal->createProperty('DTSTART', new \DateTime('2013-06-07 15:05:00'));
         $dtStart['VALUE'] = 'DATE';
 
-        $this->assertEquals("DTSTART;VALUE=DATE:20130607\r\n", $dtStart->serialize());
+        self::assertEquals("DTSTART;VALUE=DATE:20130607\r\n", $dtStart->serialize());
     }
 
     public function testValidate(): void
     {
         $exDate = $this->vcal->createProperty('EXDATE', '-00011130T143000Z');
         $messages = $exDate->validate();
-        $this->assertCount(1, $messages);
-        $this->assertEquals(3, $messages[0]['level']);
+        self::assertCount(1, $messages);
+        self::assertEquals(3, $messages[0]['level']);
     }
 
     /**
@@ -342,6 +342,6 @@ public function testCreateDatePropertyThroughAdd(): void
             ['VALUE' => 'DATE']
         );
 
-        $this->assertEquals("DTSTART;VALUE=DATE:20140307\r\n", $dtstart->serialize());
+        self::assertEquals("DTSTART;VALUE=DATE:20140307\r\n", $dtstart->serialize());
     }
 }
diff --git a/tests/VObject/Property/ICalendar/DurationTest.php b/tests/VObject/Property/ICalendar/DurationTest.php
index 5c90cfb1..044f8627 100644
--- a/tests/VObject/Property/ICalendar/DurationTest.php
+++ b/tests/VObject/Property/ICalendar/DurationTest.php
@@ -12,7 +12,7 @@ public function testGetDateInterval(): void
         $vcal = new VCalendar();
         $event = $vcal->add('VEVENT', ['DURATION' => ['PT1H']]);
 
-        $this->assertEquals(
+        self::assertEquals(
             new \DateInterval('PT1H'),
             $event->{'DURATION'}->getDateInterval()
         );
diff --git a/tests/VObject/Property/ICalendar/RecurTest.php b/tests/VObject/Property/ICalendar/RecurTest.php
index e0cce696..b2750f03 100644
--- a/tests/VObject/Property/ICalendar/RecurTest.php
+++ b/tests/VObject/Property/ICalendar/RecurTest.php
@@ -16,12 +16,12 @@ public function testParts(): void
         $vcal = new VCalendar();
         $recur = $vcal->add('RRULE', 'FREQ=Daily');
 
-        $this->assertInstanceOf(Recur::class, $recur);
+        self::assertInstanceOf(Recur::class, $recur);
 
-        $this->assertEquals(['FREQ' => 'DAILY'], $recur->getParts());
+        self::assertEquals(['FREQ' => 'DAILY'], $recur->getParts());
         $recur->setParts(['freq' => 'MONTHLY']);
 
-        $this->assertEquals(['FREQ' => 'MONTHLY'], $recur->getParts());
+        self::assertEquals(['FREQ' => 'MONTHLY'], $recur->getParts());
     }
 
     public function testSetValueBadVal(): void
@@ -37,7 +37,7 @@ public function testSetValueWithCount(): void
         $vcal = new VCalendar();
         $recur = $vcal->add('RRULE', 'FREQ=Daily');
         $recur->setValue(['COUNT' => 3]);
-        $this->assertEquals(3, $recur->getParts()['COUNT']);
+        self::assertEquals(3, $recur->getParts()['COUNT']);
     }
 
     public function testGetJSONWithCount(): void
@@ -58,8 +58,8 @@ public function testGetJSONWithCount(): void
         $vcal = Reader::read($input);
         $rrule = $vcal->VEVENT->RRULE;
         $count = $rrule->getJsonValue()[0]['count'];
-        $this->assertTrue(is_int($count));
-        $this->assertEquals(3, $count);
+        self::assertTrue(is_int($count));
+        self::assertEquals(3, $count);
     }
 
     public function testSetSubParts(): void
@@ -67,7 +67,7 @@ public function testSetSubParts(): void
         $vcal = new VCalendar();
         $recur = $vcal->add('RRULE', ['FREQ' => 'DAILY', 'BYDAY' => 'mo,tu', 'BYMONTH' => [0, 1]]);
 
-        $this->assertEquals([
+        self::assertEquals([
             'FREQ' => 'DAILY',
             'BYDAY' => ['MO', 'TU'],
             'BYMONTH' => [0, 1],
@@ -92,7 +92,7 @@ public function testGetJSONWithUntil(): void
         $vcal = Reader::read($input);
         $rrule = $vcal->VEVENT->RRULE;
         $untilJsonString = $rrule->getJsonValue()[0]['until'];
-        $this->assertEquals('2016-03-05T23:00:00Z', $untilJsonString);
+        self::assertEquals('2016-03-05T23:00:00Z', $untilJsonString);
     }
 
     public function testValidateStripEmpties(): void
@@ -114,11 +114,11 @@ public function testValidateStripEmpties(): void
 ';
 
         $vcal = Reader::read($input);
-        $this->assertCount(
+        self::assertCount(
             1,
             $vcal->validate()
         );
-        $this->assertCount(
+        self::assertCount(
             1,
             $vcal->validate($vcal::REPAIR)
         );
@@ -139,7 +139,7 @@ public function testValidateStripEmpties(): void
 END:VCALENDAR
 ';
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $vcal
         );
@@ -164,11 +164,11 @@ public function testValidateStripNoFreq(): void
 ';
 
         $vcal = Reader::read($input);
-        $this->assertCount(
+        self::assertCount(
             1,
             $vcal->validate()
         );
-        $this->assertCount(
+        self::assertCount(
             1,
             $vcal->validate($vcal::REPAIR)
         );
@@ -188,7 +188,7 @@ public function testValidateStripNoFreq(): void
 END:VCALENDAR
 ';
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $vcal
         );
@@ -200,10 +200,10 @@ public function testValidateInvalidByMonthRruleWithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=0');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(1, $result);
-        $this->assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24', $property->getValue());
+        self::assertCount(1, $result);
+        self::assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24', $property->getValue());
     }
 
     public function testValidateInvalidByMonthRruleWithoutRepair(): void
@@ -212,10 +212,10 @@ public function testValidateInvalidByMonthRruleWithoutRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=0');
         $result = $property->validate();
 
-        $this->assertCount(1, $result);
-        $this->assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
-        $this->assertEquals(3, $result[0]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=0', $property->getValue());
+        self::assertCount(1, $result);
+        self::assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
+        self::assertEquals(3, $result[0]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=0', $property->getValue());
     }
 
     public function testValidateInvalidByMonthRruleWithRepair2(): void
@@ -224,10 +224,10 @@ public function testValidateInvalidByMonthRruleWithRepair2(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=bla');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(1, $result);
-        $this->assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24', $property->getValue());
+        self::assertCount(1, $result);
+        self::assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24', $property->getValue());
     }
 
     public function testValidateInvalidByMonthRruleWithoutRepair2(): void
@@ -236,11 +236,11 @@ public function testValidateInvalidByMonthRruleWithoutRepair2(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=bla');
         $result = $property->validate();
 
-        $this->assertCount(1, $result);
-        $this->assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
-        $this->assertEquals(3, $result[0]['level']);
+        self::assertCount(1, $result);
+        self::assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
+        self::assertEquals(3, $result[0]['level']);
         // Without repair the invalid BYMONTH is still there, but the value is changed to uppercase
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=BLA', $property->getValue());
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=BLA', $property->getValue());
     }
 
     public function testValidateInvalidByMonthRruleValue14WithRepair(): void
@@ -249,10 +249,10 @@ public function testValidateInvalidByMonthRruleValue14WithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=14');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(1, $result);
-        $this->assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24', $property->getValue());
+        self::assertCount(1, $result);
+        self::assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24', $property->getValue());
     }
 
     public function testValidateInvalidByMonthRruleMultipleWithRepair(): void
@@ -261,12 +261,12 @@ public function testValidateInvalidByMonthRruleMultipleWithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=0,1,2,3,4,14');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(2, $result);
-        $this->assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
-        $this->assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[1]['message']);
-        $this->assertEquals(1, $result[1]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=1,2,3,4', $property->getValue());
+        self::assertCount(2, $result);
+        self::assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
+        self::assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[1]['message']);
+        self::assertEquals(1, $result[1]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=1,2,3,4', $property->getValue());
     }
 
     public function testValidateOneOfManyInvalidByMonthRruleWithRepair(): void
@@ -275,19 +275,19 @@ public function testValidateOneOfManyInvalidByMonthRruleWithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=bla,3,foo');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(2, $result);
-        $this->assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
-        $this->assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[1]['message']);
-        $this->assertEquals(1, $result[1]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=3', $property->getValue());
+        self::assertCount(2, $result);
+        self::assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
+        self::assertEquals('BYMONTH in RRULE must have value(s) between 1 and 12!', $result[1]['message']);
+        self::assertEquals(1, $result[1]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=3', $property->getValue());
     }
 
     public function testValidateValidByMonthRrule(): void
     {
         $calendar = new VCalendar();
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=2,3');
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=2,3', $property->getValue());
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYMONTHDAY=24;BYMONTH=2,3', $property->getValue());
     }
 
     /**
@@ -300,8 +300,8 @@ public function testValidateRruleBySecondZero(): void
         $result = $property->validate(Node::REPAIR);
 
         // There should be 0 warnings and the value should be unchanged
-        $this->assertEmpty($result);
-        $this->assertEquals('FREQ=DAILY;BYHOUR=10;BYMINUTE=30;BYSECOND=0;UNTIL=20150616T153000Z', $property->getValue());
+        self::assertEmpty($result);
+        self::assertEquals('FREQ=DAILY;BYHOUR=10;BYMINUTE=30;BYSECOND=0;UNTIL=20150616T153000Z', $property->getValue());
     }
 
     public function testValidateValidByWeekNoWithRepair(): void
@@ -310,8 +310,8 @@ public function testValidateValidByWeekNoWithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYWEEKNO=11');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(0, $result);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYWEEKNO=11', $property->getValue());
+        self::assertCount(0, $result);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYWEEKNO=11', $property->getValue());
     }
 
     public function testValidateInvalidByWeekNoWithRepair(): void
@@ -320,10 +320,10 @@ public function testValidateInvalidByWeekNoWithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYWEEKNO=55;BYDAY=WE');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(1, $result);
-        $this->assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYDAY=WE', $property->getValue());
+        self::assertCount(1, $result);
+        self::assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYDAY=WE', $property->getValue());
     }
 
     public function testValidateMultipleInvalidByWeekNoWithRepair(): void
@@ -332,12 +332,12 @@ public function testValidateMultipleInvalidByWeekNoWithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYWEEKNO=55,2,-80;BYDAY=WE');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(2, $result);
-        $this->assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
-        $this->assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[1]['message']);
-        $this->assertEquals(1, $result[1]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYWEEKNO=2;BYDAY=WE', $property->getValue());
+        self::assertCount(2, $result);
+        self::assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
+        self::assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[1]['message']);
+        self::assertEquals(1, $result[1]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYWEEKNO=2;BYDAY=WE', $property->getValue());
     }
 
     public function testValidateAllInvalidByWeekNoWithRepair(): void
@@ -346,12 +346,12 @@ public function testValidateAllInvalidByWeekNoWithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYWEEKNO=55,-80;BYDAY=WE');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(2, $result);
-        $this->assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
-        $this->assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[1]['message']);
-        $this->assertEquals(1, $result[1]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYDAY=WE', $property->getValue());
+        self::assertCount(2, $result);
+        self::assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
+        self::assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[1]['message']);
+        self::assertEquals(1, $result[1]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYDAY=WE', $property->getValue());
     }
 
     public function testValidateInvalidByWeekNoWithoutRepair(): void
@@ -360,10 +360,10 @@ public function testValidateInvalidByWeekNoWithoutRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYWEEKNO=55;BYDAY=WE');
         $result = $property->validate();
 
-        $this->assertCount(1, $result);
-        $this->assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[0]['message']);
-        $this->assertEquals(3, $result[0]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYWEEKNO=55;BYDAY=WE', $property->getValue());
+        self::assertCount(1, $result);
+        self::assertEquals('BYWEEKNO in RRULE must have value(s) from -53 to -1, or 1 to 53!', $result[0]['message']);
+        self::assertEquals(3, $result[0]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYWEEKNO=55;BYDAY=WE', $property->getValue());
     }
 
     public function testValidateValidByYearDayWithRepair(): void
@@ -372,8 +372,8 @@ public function testValidateValidByYearDayWithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYYEARDAY=119');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(0, $result);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYYEARDAY=119', $property->getValue());
+        self::assertCount(0, $result);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYYEARDAY=119', $property->getValue());
     }
 
     public function testValidateInvalidByYearDayWithRepair(): void
@@ -382,10 +382,10 @@ public function testValidateInvalidByYearDayWithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYYEARDAY=367;BYDAY=WE');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(1, $result);
-        $this->assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYDAY=WE', $property->getValue());
+        self::assertCount(1, $result);
+        self::assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYDAY=WE', $property->getValue());
     }
 
     public function testValidateMultipleInvalidByYearDayWithRepair(): void
@@ -394,12 +394,12 @@ public function testValidateMultipleInvalidByYearDayWithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYYEARDAY=380,2,-390;BYDAY=WE');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(2, $result);
-        $this->assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
-        $this->assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[1]['message']);
-        $this->assertEquals(1, $result[1]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYYEARDAY=2;BYDAY=WE', $property->getValue());
+        self::assertCount(2, $result);
+        self::assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
+        self::assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[1]['message']);
+        self::assertEquals(1, $result[1]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYYEARDAY=2;BYDAY=WE', $property->getValue());
     }
 
     public function testValidateAllInvalidByYearDayWithRepair(): void
@@ -408,12 +408,12 @@ public function testValidateAllInvalidByYearDayWithRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYYEARDAY=455,-480;BYDAY=WE');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertCount(2, $result);
-        $this->assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
-        $this->assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[1]['message']);
-        $this->assertEquals(1, $result[1]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYDAY=WE', $property->getValue());
+        self::assertCount(2, $result);
+        self::assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
+        self::assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[1]['message']);
+        self::assertEquals(1, $result[1]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYDAY=WE', $property->getValue());
     }
 
     public function testValidateInvalidByYearDayWithoutRepair(): void
@@ -422,9 +422,9 @@ public function testValidateInvalidByYearDayWithoutRepair(): void
         $property = $calendar->createProperty('RRULE', 'FREQ=YEARLY;COUNT=6;BYYEARDAY=380;BYDAY=WE');
         $result = $property->validate();
 
-        $this->assertCount(1, $result);
-        $this->assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[0]['message']);
-        $this->assertEquals(3, $result[0]['level']);
-        $this->assertEquals('FREQ=YEARLY;COUNT=6;BYYEARDAY=380;BYDAY=WE', $property->getValue());
+        self::assertCount(1, $result);
+        self::assertEquals('BYYEARDAY in RRULE must have value(s) from -366 to -1, or 1 to 366!', $result[0]['message']);
+        self::assertEquals(3, $result[0]['level']);
+        self::assertEquals('FREQ=YEARLY;COUNT=6;BYYEARDAY=380;BYDAY=WE', $property->getValue());
     }
 }
diff --git a/tests/VObject/Property/TextTest.php b/tests/VObject/Property/TextTest.php
index 98489415..0fa320bf 100644
--- a/tests/VObject/Property/TextTest.php
+++ b/tests/VObject/Property/TextTest.php
@@ -21,12 +21,12 @@ public function assertVCard21Serialization($propValue, string $expected): void
 
         $output = $doc->serialize();
 
-        $this->assertEquals("BEGIN:VCARD\r\nVERSION:2.1\r\n$expected\r\nEND:VCARD\r\n", $output);
+        self::assertEquals("BEGIN:VCARD\r\nVERSION:2.1\r\n$expected\r\nEND:VCARD\r\n", $output);
     }
 
     public function testSerializeVCard21(): void
     {
-        $this->assertVCard21Serialization(
+        self::assertVCard21Serialization(
             'f;oo',
             'PROP;P1=V1:f;oo'
         );
@@ -34,7 +34,7 @@ public function testSerializeVCard21(): void
 
     public function testSerializeVCard21Array(): void
     {
-        $this->assertVCard21Serialization(
+        self::assertVCard21Serialization(
             ['f;oo', 'bar'],
             'PROP;P1=V1:f\;oo;bar'
         );
@@ -42,7 +42,7 @@ public function testSerializeVCard21Array(): void
 
     public function testSerializeVCard21Fold(): void
     {
-        $this->assertVCard21Serialization(
+        self::assertVCard21Serialization(
             str_repeat('x', 80),
             'PROP;P1=V1:'.str_repeat('x', 64)."\r\n ".str_repeat('x', 16)
         );
@@ -50,7 +50,7 @@ public function testSerializeVCard21Fold(): void
 
     public function testSerializeQuotedPrintable(): void
     {
-        $this->assertVCard21Serialization(
+        self::assertVCard21Serialization(
             "foo\r\nbar",
             'PROP;P1=V1;ENCODING=QUOTED-PRINTABLE:foo=0D=0Abar'
         );
@@ -58,7 +58,7 @@ public function testSerializeQuotedPrintable(): void
 
     public function testSerializeQuotedPrintableFold(): void
     {
-        $this->assertVCard21Serialization(
+        self::assertVCard21Serialization(
             "foo\r\nbarxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
             "PROP;P1=V1;ENCODING=QUOTED-PRINTABLE:foo=0D=0Abarxxxxxxxxxxxxxxxxxxxxxxxxxx=\r\n xxx"
         );
@@ -76,12 +76,12 @@ public function testValidateMinimumPropValue(): void
 IN;
 
         $vcard = \Sabre\VObject\Reader::read($vcard);
-        $this->assertCount(1, $vcard->validate());
+        self::assertCount(1, $vcard->validate());
 
-        $this->assertCount(1, $vcard->N->getParts());
+        self::assertCount(1, $vcard->N->getParts());
 
         $vcard->validate(\Sabre\VObject\Node::REPAIR);
 
-        $this->assertCount(5, $vcard->N->getParts());
+        self::assertCount(5, $vcard->N->getParts());
     }
 }
diff --git a/tests/VObject/Property/UriTest.php b/tests/VObject/Property/UriTest.php
index 80810104..edb7cc39 100644
--- a/tests/VObject/Property/UriTest.php
+++ b/tests/VObject/Property/UriTest.php
@@ -21,6 +21,6 @@ public function testAlwaysEncodeUriVCalendar(): void
 END:VCALENDAR
 ICS;
         $output = Reader::read($input)->serialize();
-        $this->assertStringContainsString('URL;VALUE=URI:http://example.org/', $output);
+        self::assertStringContainsString('URL;VALUE=URI:http://example.org/', $output);
     }
 }
diff --git a/tests/VObject/Property/VCard/DateAndOrTimeTest.php b/tests/VObject/Property/VCard/DateAndOrTimeTest.php
index 9aefefb9..8e2cb320 100644
--- a/tests/VObject/Property/VCard/DateAndOrTimeTest.php
+++ b/tests/VObject/Property/VCard/DateAndOrTimeTest.php
@@ -16,7 +16,7 @@ public function testGetJsonValue(string $input, string $output): void
         $vcard = new VObject\Component\VCard();
         $prop = $vcard->createProperty('BDAY', $input);
 
-        $this->assertEquals([$output], $prop->getJsonValue());
+        self::assertEquals([$output], $prop->getJsonValue());
     }
 
     public function dates(): array
@@ -90,7 +90,7 @@ public function testSetParts(): void
             new \DateTime('2014-04-02 18:37:00'),
         ]);
 
-        $this->assertEquals('20140402T183700Z', $prop->getValue());
+        self::assertEquals('20140402T183700Z', $prop->getValue());
     }
 
     public function testSetPartsDateTimeImmutable(): void
@@ -102,7 +102,7 @@ public function testSetPartsDateTimeImmutable(): void
             new \DateTimeImmutable('2014-04-02 18:37:00'),
         ]);
 
-        $this->assertEquals('20140402T183700Z', $prop->getValue());
+        self::assertEquals('20140402T183700Z', $prop->getValue());
     }
 
     public function testSetPartsTooMany(): void
@@ -126,7 +126,7 @@ public function testSetPartsString(): void
             '20140402T183700Z',
         ]);
 
-        $this->assertEquals('20140402T183700Z', $prop->getValue());
+        self::assertEquals('20140402T183700Z', $prop->getValue());
     }
 
     public function testSetValueDateTime(): void
@@ -138,7 +138,7 @@ public function testSetValueDateTime(): void
             new \DateTime('2014-04-02 18:37:00')
         );
 
-        $this->assertEquals('20140402T183700Z', $prop->getValue());
+        self::assertEquals('20140402T183700Z', $prop->getValue());
     }
 
     public function testSetValueDateTimeImmutable(): void
@@ -150,7 +150,7 @@ public function testSetValueDateTimeImmutable(): void
             new \DateTimeImmutable('2014-04-02 18:37:00')
         );
 
-        $this->assertEquals('20140402T183700Z', $prop->getValue());
+        self::assertEquals('20140402T183700Z', $prop->getValue());
     }
 
     public function testSetDateTimeOffset(): void
@@ -162,7 +162,7 @@ public function testSetDateTimeOffset(): void
             new \DateTime('2014-04-02 18:37:00', new \DateTimeZone('America/Toronto'))
         );
 
-        $this->assertEquals('20140402T183700-0400', $prop->getValue());
+        self::assertEquals('20140402T183700-0400', $prop->getValue());
     }
 
     public function testGetDateTime(): void
@@ -173,7 +173,7 @@ public function testGetDateTime(): void
         $prop = $vcard->createProperty('BDAY', $datetime);
 
         $dt = $prop->getDateTime();
-        $this->assertEquals('2014-04-02T18:37:00-04:00', $dt->format('c'), 'For some reason this one failed. Current default timezone is: '.date_default_timezone_get());
+        self::assertEquals('2014-04-02T18:37:00-04:00', $dt->format('c'), 'For some reason this one failed. Current default timezone is: '.date_default_timezone_get());
     }
 
     public function testGetDate(): void
@@ -183,8 +183,8 @@ public function testGetDate(): void
         $vcard = new VObject\Component\VCard();
         $prop = $vcard->createProperty('BDAY', $datetime, null, 'DATE');
 
-        $this->assertEquals('DATE', $prop->getValueType());
-        $this->assertEquals('BDAY:20140402', rtrim($prop->serialize()));
+        self::assertEquals('DATE', $prop->getValueType());
+        self::assertEquals('BDAY:20140402', rtrim($prop->serialize()));
     }
 
     public function testGetDateIncomplete(): void
@@ -202,7 +202,7 @@ public function testGetDateIncomplete(): void
         $current = new \DateTime('now');
         $year = $current->format('Y');
 
-        $this->assertEquals($year.'0407', $dt->format('Ymd'));
+        self::assertEquals($year.'0407', $dt->format('Ymd'));
     }
 
     public function testGetDateIncompleteFromVCard(): void
@@ -224,7 +224,7 @@ public function testGetDateIncompleteFromVCard(): void
         $current = new \DateTime('now');
         $year = $current->format('Y');
 
-        $this->assertEquals($year.'0407', $dt->format('Ymd'));
+        self::assertEquals($year.'0407', $dt->format('Ymd'));
     }
 
     public function testValidate(): void
@@ -234,7 +234,7 @@ public function testValidate(): void
         $vcard = new VObject\Component\VCard();
         $prop = $vcard->add('BDAY', $datetime);
 
-        $this->assertEquals([], $prop->validate());
+        self::assertEquals([], $prop->validate());
     }
 
     public function testValidateBroken(): void
@@ -244,7 +244,7 @@ public function testValidateBroken(): void
         $vcard = new VObject\Component\VCard();
         $prop = $vcard->add('BDAY', $datetime);
 
-        $this->assertEquals([[
+        self::assertEquals([[
             'level' => 3,
             'message' => 'The supplied value (123) is not a correct DATE-AND-OR-TIME property',
             'node' => $prop,
diff --git a/tests/VObject/Property/VCard/LanguageTagTest.php b/tests/VObject/Property/VCard/LanguageTagTest.php
index 98a98908..be3d5c62 100644
--- a/tests/VObject/Property/VCard/LanguageTagTest.php
+++ b/tests/VObject/Property/VCard/LanguageTagTest.php
@@ -14,11 +14,11 @@ public function testMimeDir(): void
 
         $result = $mimeDir->parse($input);
 
-        $this->assertInstanceOf(LanguageTag::class, $result->LANG);
+        self::assertInstanceOf(LanguageTag::class, $result->LANG);
 
-        $this->assertEquals('nl', $result->LANG->getValue());
+        self::assertEquals('nl', $result->LANG->getValue());
 
-        $this->assertEquals(
+        self::assertEquals(
             $input,
             $result->serialize()
         );
@@ -31,15 +31,15 @@ public function testChangeAndSerialize(): void
 
         $result = $mimeDir->parse($input);
 
-        $this->assertInstanceOf(LanguageTag::class, $result->LANG);
+        self::assertInstanceOf(LanguageTag::class, $result->LANG);
         // This replicates what the vcard converter does and triggered a bug in
         // the past.
         $result->LANG->setValue(['de']);
 
-        $this->assertEquals('de', $result->LANG->getValue());
+        self::assertEquals('de', $result->LANG->getValue());
 
         $expected = "BEGIN:VCARD\r\nVERSION:4.0\r\nLANG:de\r\nEND:VCARD\r\n";
-        $this->assertEquals(
+        self::assertEquals(
             $expected,
             $result->serialize()
         );
diff --git a/tests/VObject/Property/VCard/PhoneNumberTest.php b/tests/VObject/Property/VCard/PhoneNumberTest.php
index 939a1b58..f9882cdf 100644
--- a/tests/VObject/Property/VCard/PhoneNumberTest.php
+++ b/tests/VObject/Property/VCard/PhoneNumberTest.php
@@ -12,8 +12,8 @@ public function testParser(): void
         $input = "BEGIN:VCARD\r\nVERSION:3.0\r\nTEL;TYPE=HOME;VALUE=PHONE-NUMBER:+1234\r\nEND:VCARD\r\n";
 
         $vCard = VObject\Reader::read($input);
-        $this->assertInstanceOf(PhoneNumber::class, $vCard->TEL);
-        $this->assertEquals('PHONE-NUMBER', $vCard->TEL->getValueType());
-        $this->assertEquals($input, $vCard->serialize());
+        self::assertInstanceOf(PhoneNumber::class, $vCard->TEL);
+        self::assertEquals('PHONE-NUMBER', $vCard->TEL->getValueType());
+        self::assertEquals($input, $vCard->serialize());
     }
 }
diff --git a/tests/VObject/PropertyTest.php b/tests/VObject/PropertyTest.php
index ca5033a9..b15e06b2 100644
--- a/tests/VObject/PropertyTest.php
+++ b/tests/VObject/PropertyTest.php
@@ -13,10 +13,10 @@ public function testToString(): void
         $cal = new VCalendar();
 
         $property = $cal->createProperty('propname', 'propvalue');
-        $this->assertEquals('PROPNAME', $property->name);
-        $this->assertEquals('propvalue', $property->__toString());
-        $this->assertEquals('propvalue', (string) $property);
-        $this->assertEquals('propvalue', $property->getValue());
+        self::assertEquals('PROPNAME', $property->name);
+        self::assertEquals('propvalue', $property->__toString());
+        self::assertEquals('propvalue', (string) $property);
+        self::assertEquals('propvalue', $property->getValue());
     }
 
     public function testCreate(): void
@@ -30,8 +30,8 @@ public function testCreate(): void
 
         $property = $cal->createProperty('propname', 'propvalue', $params);
 
-        $this->assertEquals('value1', $property['param1']->getValue());
-        $this->assertEquals('value2', $property['param2']->getValue());
+        self::assertEquals('value1', $property['param1']->getValue());
+        self::assertEquals('value2', $property['param2']->getValue());
     }
 
     public function testSetValue(): void
@@ -41,8 +41,8 @@ public function testSetValue(): void
         $property = $cal->createProperty('propname', 'propvalue');
         $property->setValue('value2');
 
-        $this->assertEquals('PROPNAME', $property->name);
-        $this->assertEquals('value2', $property->__toString());
+        self::assertEquals('PROPNAME', $property->name);
+        self::assertEquals('value2', $property->__toString());
     }
 
     public function testParameterExists(): void
@@ -51,9 +51,9 @@ public function testParameterExists(): void
         $property = $cal->createProperty('propname', 'propvalue');
         $property['paramname'] = 'paramvalue';
 
-        $this->assertTrue(isset($property['PARAMNAME']));
-        $this->assertTrue(isset($property['paramname']));
-        $this->assertFalse(isset($property['foo']));
+        self::assertTrue(isset($property['PARAMNAME']));
+        self::assertTrue(isset($property['paramname']));
+        self::assertFalse(isset($property['foo']));
     }
 
     public function testParameterGet(): void
@@ -62,7 +62,7 @@ public function testParameterGet(): void
         $property = $cal->createProperty('propname', 'propvalue');
         $property['paramname'] = 'paramvalue';
 
-        $this->assertInstanceOf(Parameter::class, $property['paramname']);
+        self::assertInstanceOf(Parameter::class, $property['paramname']);
     }
 
     public function testParameterNotExists(): void
@@ -71,7 +71,7 @@ public function testParameterNotExists(): void
         $property = $cal->createProperty('propname', 'propvalue');
         $property['paramname'] = 'paramvalue';
 
-        $this->assertNull($property['foo']);
+        self::assertNull($property['foo']);
     }
 
     public function testParameterMultiple(): void
@@ -81,8 +81,8 @@ public function testParameterMultiple(): void
         $property['paramname'] = 'paramvalue';
         $property->add('paramname', 'paramvalue');
 
-        $this->assertInstanceOf(Parameter::class, $property['paramname']);
-        $this->assertCount(2, $property['paramname']->getParts());
+        self::assertInstanceOf(Parameter::class, $property['paramname']);
+        self::assertCount(2, $property['paramname']->getParts());
     }
 
     public function testSetParameterAsString(): void
@@ -91,10 +91,10 @@ public function testSetParameterAsString(): void
         $property = $cal->createProperty('propname', 'propvalue');
         $property['paramname'] = 'paramvalue';
 
-        $this->assertCount(1, $property->parameters());
-        $this->assertInstanceOf(Parameter::class, $property->parameters['PARAMNAME']);
-        $this->assertEquals('PARAMNAME', $property->parameters['PARAMNAME']->name);
-        $this->assertEquals('paramvalue', $property->parameters['PARAMNAME']->getValue());
+        self::assertCount(1, $property->parameters());
+        self::assertInstanceOf(Parameter::class, $property->parameters['PARAMNAME']);
+        self::assertEquals('PARAMNAME', $property->parameters['PARAMNAME']->name);
+        self::assertEquals('paramvalue', $property->parameters['PARAMNAME']->getValue());
     }
 
     public function testUnsetParameter(): void
@@ -104,7 +104,7 @@ public function testUnsetParameter(): void
         $property['paramname'] = 'paramvalue';
 
         unset($property['PARAMNAME']);
-        $this->assertCount(0, $property->parameters());
+        self::assertCount(0, $property->parameters());
     }
 
     public function testSerialize(): void
@@ -112,7 +112,7 @@ public function testSerialize(): void
         $cal = new VCalendar();
         $property = $cal->createProperty('propname', 'propvalue');
 
-        $this->assertEquals("PROPNAME:propvalue\r\n", $property->serialize());
+        self::assertEquals("PROPNAME:propvalue\r\n", $property->serialize());
     }
 
     public function testSerializeParam(): void
@@ -123,7 +123,7 @@ public function testSerializeParam(): void
             'paramname2' => 'paramvalue2',
         ]);
 
-        $this->assertEquals("PROPNAME;PARAMNAME=paramvalue;PARAMNAME2=paramvalue2:propvalue\r\n", $property->serialize());
+        self::assertEquals("PROPNAME;PARAMNAME=paramvalue;PARAMNAME2=paramvalue2:propvalue\r\n", $property->serialize());
     }
 
     public function testSerializeNewLine(): void
@@ -131,7 +131,7 @@ public function testSerializeNewLine(): void
         $cal = new VCalendar();
         $property = $cal->createProperty('SUMMARY', "line1\nline2");
 
-        $this->assertEquals("SUMMARY:line1\\nline2\r\n", $property->serialize());
+        self::assertEquals("SUMMARY:line1\\nline2\r\n", $property->serialize());
     }
 
     public function testSerializeLongLine(): void
@@ -142,7 +142,7 @@ public function testSerializeLongLine(): void
 
         $expected = 'PROPNAME:'.str_repeat('!', 66)."\r\n ".str_repeat('!', 74)."\r\n ".str_repeat('!', 60)."\r\n";
 
-        $this->assertEquals($expected, $property->serialize());
+        self::assertEquals($expected, $property->serialize());
     }
 
     public function testSerializeUTF8LineFold(): void
@@ -156,7 +156,7 @@ public function testSerializeUTF8LineFold(): void
         //  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! (" "         + 73x"!" = 74 bytes)
         //  äfoo
         $expected = 'PROPNAME:'.str_repeat('!', 65)."\r\n \xc3\xa4bla".str_repeat('!', 69)."\r\n ".str_repeat('!', 73)."\r\n \xc3\xa4foo\r\n";
-        $this->assertEquals($expected, $property->serialize());
+        self::assertEquals($expected, $property->serialize());
     }
 
     public function testGetIterator(): void
@@ -165,7 +165,7 @@ public function testGetIterator(): void
         $it = new ElementList([]);
         $property = $cal->createProperty('propname', 'propvalue');
         $property->setIterator($it);
-        $this->assertEquals($it, $property->getIterator());
+        self::assertEquals($it, $property->getIterator());
     }
 
     public function testGetIteratorDefault(): void
@@ -173,8 +173,8 @@ public function testGetIteratorDefault(): void
         $cal = new VCalendar();
         $property = $cal->createProperty('propname', 'propvalue');
         $it = $property->getIterator();
-        $this->assertTrue($it instanceof ElementList);
-        $this->assertCount(1, $it);
+        self::assertTrue($it instanceof ElementList);
+        self::assertCount(1, $it);
     }
 
     public function testAddScalar(): void
@@ -184,11 +184,11 @@ public function testAddScalar(): void
 
         $property->add('myparam', 'value');
 
-        $this->assertCount(1, $property->parameters());
+        self::assertCount(1, $property->parameters());
 
-        $this->assertTrue($property->parameters['MYPARAM'] instanceof Parameter);
-        $this->assertEquals('MYPARAM', $property->parameters['MYPARAM']->name);
-        $this->assertEquals('value', $property->parameters['MYPARAM']->getValue());
+        self::assertTrue($property->parameters['MYPARAM'] instanceof Parameter);
+        self::assertEquals('MYPARAM', $property->parameters['MYPARAM']->name);
+        self::assertEquals('value', $property->parameters['MYPARAM']->getValue());
     }
 
     public function testAddParameter(): void
@@ -198,8 +198,8 @@ public function testAddParameter(): void
 
         $prop->add('MYPARAM', 'value');
 
-        $this->assertCount(1, $prop->parameters());
-        $this->assertEquals('MYPARAM', $prop['myparam']->name);
+        self::assertCount(1, $prop->parameters());
+        self::assertEquals('MYPARAM', $prop['myparam']->name);
     }
 
     public function testAddParameterTwice(): void
@@ -210,10 +210,10 @@ public function testAddParameterTwice(): void
         $prop->add('MYPARAM', 'value1');
         $prop->add('MYPARAM', 'value2');
 
-        $this->assertCount(1, $prop->parameters);
-        $this->assertCount(2, $prop->parameters['MYPARAM']->getParts());
+        self::assertCount(1, $prop->parameters);
+        self::assertCount(2, $prop->parameters['MYPARAM']->getParts());
 
-        $this->assertEquals('MYPARAM', $prop['MYPARAM']->name);
+        self::assertEquals('MYPARAM', $prop['MYPARAM']->name);
     }
 
     public function testClone(): void
@@ -225,7 +225,7 @@ public function testClone(): void
         $property2 = clone $property;
 
         $property['FOO'] = 'BAZ';
-        $this->assertEquals('BAR', (string) $property2['FOO']);
+        self::assertEquals('BAR', (string) $property2['FOO']);
     }
 
     public function testCreateParams(): void
@@ -236,8 +236,8 @@ public function testCreateParams(): void
             'param2' => ['value2', 'value3'],
         ]);
 
-        $this->assertCount(1, $property['PARAM1']->getParts());
-        $this->assertCount(2, $property['PARAM2']->getParts());
+        self::assertCount(1, $property['PARAM1']->getParts());
+        self::assertCount(2, $property['PARAM2']->getParts());
     }
 
     public function testValidateNonUTF8(): void
@@ -246,8 +246,8 @@ public function testValidateNonUTF8(): void
         $property = $calendar->createProperty('X-PROP', "Bla\x00");
         $result = $property->validate(Property::REPAIR);
 
-        $this->assertEquals('Property contained a control character (0x00)', $result[0]['message']);
-        $this->assertEquals('Bla', $property->getValue());
+        self::assertEquals('Property contained a control character (0x00)', $result[0]['message']);
+        self::assertEquals('Bla', $property->getValue());
     }
 
     public function testValidateControlChars(): void
@@ -268,8 +268,8 @@ public function testValidateControlChars(): void
         $property = $calendar->createProperty('X-PROP', $s);
         $result = $property->validate(Property::REPAIR);
 
-        $this->assertEquals('Property contained a control character (0x7f)', $result[0]['message']);
-        $this->assertEquals("chars[7F()5E(^)5C(\\\\)3B(\\;)3A(:)2C(\\,)22(\")20( )1F()1E()1D()1C()1B()1A()19()18()17()16()15()14()13()12()11()10()0F()0E()0D()0C()0B()0A(\\n)09(\t)08()07()06()05()04()03()02()01()00()]end", $property->getRawMimeDirValue());
+        self::assertEquals('Property contained a control character (0x7f)', $result[0]['message']);
+        self::assertEquals("chars[7F()5E(^)5C(\\\\)3B(\\;)3A(:)2C(\\,)22(\")20( )1F()1E()1D()1C()1B()1A()19()18()17()16()15()14()13()12()11()10()0F()0E()0D()0C()0B()0A(\\n)09(\t)08()07()06()05()04()03()02()01()00()]end", $property->getRawMimeDirValue());
     }
 
     /**
@@ -281,8 +281,8 @@ public function testValidateBadPropertyName(): void
         $property = $calendar->createProperty('X_*&PROP*', 'Bla');
         $result = $property->validate(Node::REPAIR);
 
-        $this->assertEquals('The property name: X_*&PROP* contains invalid characters. Only A-Z, 0-9 and - are allowed', $result[0]['message']);
-        $this->assertEquals('X-PROP', $property->name);
+        self::assertEquals('The property name: X_*&PROP* contains invalid characters. Only A-Z, 0-9 and - are allowed', $result[0]['message']);
+        self::assertEquals('X-PROP', $property->name);
     }
 
     /**
@@ -292,24 +292,24 @@ public function testGetValue(): void
     {
         $calendar = new VCalendar();
         $property = $calendar->createProperty('SUMMARY', null);
-        $this->assertEquals([], $property->getParts());
-        $this->assertNull($property->getValue());
+        self::assertEquals([], $property->getParts());
+        self::assertNull($property->getValue());
 
         $property->setValue([]);
-        $this->assertEquals([], $property->getParts());
-        $this->assertNull($property->getValue());
+        self::assertEquals([], $property->getParts());
+        self::assertNull($property->getValue());
 
         $property->setValue([1]);
-        $this->assertEquals([1], $property->getParts());
-        $this->assertEquals(1, $property->getValue());
+        self::assertEquals([1], $property->getParts());
+        self::assertEquals(1, $property->getValue());
 
         $property->setValue([1, 2]);
-        $this->assertEquals([1, 2], $property->getParts());
-        $this->assertEquals('1,2', $property->getValue());
+        self::assertEquals([1, 2], $property->getParts());
+        self::assertEquals('1,2', $property->getValue());
 
         $property->setValue('str');
-        $this->assertEquals(['str'], $property->getParts());
-        $this->assertEquals('str', $property->getValue());
+        self::assertEquals(['str'], $property->getParts());
+        self::assertEquals('str', $property->getValue());
     }
 
     /**
@@ -346,8 +346,8 @@ public function testValidateBadEncoding(): void
 
         $result = $property->validate();
 
-        $this->assertEquals('ENCODING=INVALID is not valid for this document type.', $result[0]['message']);
-        $this->assertEquals(3, $result[0]['level']);
+        self::assertEquals('ENCODING=INVALID is not valid for this document type.', $result[0]['message']);
+        self::assertEquals(3, $result[0]['level']);
     }
 
     public function testValidateBadEncodingVCard4(): void
@@ -358,8 +358,8 @@ public function testValidateBadEncodingVCard4(): void
 
         $result = $property->validate();
 
-        $this->assertEquals('ENCODING parameter is not valid in vCard 4.', $result[0]['message']);
-        $this->assertEquals(3, $result[0]['level']);
+        self::assertEquals('ENCODING parameter is not valid in vCard 4.', $result[0]['message']);
+        self::assertEquals(3, $result[0]['level']);
     }
 
     public function testValidateBadEncodingVCard3(): void
@@ -370,14 +370,14 @@ public function testValidateBadEncodingVCard3(): void
 
         $result = $property->validate();
 
-        $this->assertEquals('ENCODING=BASE64 is not valid for this document type.', $result[0]['message']);
-        $this->assertEquals(3, $result[0]['level']);
+        self::assertEquals('ENCODING=BASE64 is not valid for this document type.', $result[0]['message']);
+        self::assertEquals(3, $result[0]['level']);
 
         // Validate the reparation of BASE64 formatted vCard v3
         $result = $property->validate(Property::REPAIR);
 
-        $this->assertEquals('ENCODING=BASE64 has been transformed to ENCODING=B.', $result[0]['message']);
-        $this->assertEquals(1, $result[0]['level']);
+        self::assertEquals('ENCODING=BASE64 has been transformed to ENCODING=B.', $result[0]['message']);
+        self::assertEquals(1, $result[0]['level']);
     }
 
     public function testValidateBadEncodingVCard21(): void
@@ -388,7 +388,7 @@ public function testValidateBadEncodingVCard21(): void
 
         $result = $property->validate();
 
-        $this->assertEquals('ENCODING=B is not valid for this document type.', $result[0]['message']);
-        $this->assertEquals(3, $result[0]['level']);
+        self::assertEquals('ENCODING=B is not valid for this document type.', $result[0]['message']);
+        self::assertEquals(3, $result[0]['level']);
     }
 }
diff --git a/tests/VObject/ReaderTest.php b/tests/VObject/ReaderTest.php
index dcdd6b0d..645ed181 100644
--- a/tests/VObject/ReaderTest.php
+++ b/tests/VObject/ReaderTest.php
@@ -12,9 +12,9 @@ public function testReadComponent(): void
 
         $result = Reader::read($data);
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCALENDAR', $result->name);
-        $this->assertCount(0, $result->children());
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCALENDAR', $result->name);
+        self::assertCount(0, $result->children());
     }
 
     public function testReadStream(): void
@@ -27,9 +27,9 @@ public function testReadStream(): void
 
         $result = Reader::read($stream);
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCALENDAR', $result->name);
-        $this->assertCount(0, $result->children());
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCALENDAR', $result->name);
+        self::assertCount(0, $result->children());
     }
 
     public function testReadComponentUnixNewLine(): void
@@ -38,9 +38,9 @@ public function testReadComponentUnixNewLine(): void
 
         $result = Reader::read($data);
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCALENDAR', $result->name);
-        $this->assertCount(0, $result->children());
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCALENDAR', $result->name);
+        self::assertCount(0, $result->children());
     }
 
     public function testReadComponentLineFold(): void
@@ -49,9 +49,9 @@ public function testReadComponentLineFold(): void
 
         $result = Reader::read($data);
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCALENDAR', $result->name);
-        $this->assertCount(0, $result->children());
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCALENDAR', $result->name);
+        self::assertCount(0, $result->children());
     }
 
     public function testReadCorruptComponent(): void
@@ -76,9 +76,9 @@ public function testReadProperty(): void
         $result = Reader::read($data);
 
         $result = $result->SUMMARY;
-        $this->assertInstanceOf(Property::class, $result);
-        $this->assertEquals('SUMMARY', $result->name);
-        $this->assertEquals('propValue', $result->getValue());
+        self::assertInstanceOf(Property::class, $result);
+        self::assertEquals('SUMMARY', $result->name);
+        self::assertEquals('propValue', $result->getValue());
     }
 
     public function testReadPropertyWithNewLine(): void
@@ -87,9 +87,9 @@ public function testReadPropertyWithNewLine(): void
         $result = Reader::read($data);
 
         $result = $result->SUMMARY;
-        $this->assertInstanceOf(Property::class, $result);
-        $this->assertEquals('SUMMARY', $result->name);
-        $this->assertEquals("Line1\nLine2\nLine3\\Not the 4th line!", $result->getValue());
+        self::assertInstanceOf(Property::class, $result);
+        self::assertEquals('SUMMARY', $result->name);
+        self::assertEquals("Line1\nLine2\nLine3\\Not the 4th line!", $result->getValue());
     }
 
     public function testReadMappedProperty(): void
@@ -98,9 +98,9 @@ public function testReadMappedProperty(): void
         $result = Reader::read($data);
 
         $result = $result->DTSTART;
-        $this->assertInstanceOf(Property\ICalendar\DateTime::class, $result);
-        $this->assertEquals('DTSTART', $result->name);
-        $this->assertEquals('20110529', $result->getValue());
+        self::assertInstanceOf(Property\ICalendar\DateTime::class, $result);
+        self::assertEquals('DTSTART', $result->name);
+        self::assertEquals('20110529', $result->getValue());
     }
 
     public function testReadMappedPropertyGrouped(): void
@@ -109,21 +109,21 @@ public function testReadMappedPropertyGrouped(): void
         $result = Reader::read($data);
 
         $result = $result->DTSTART;
-        $this->assertInstanceOf(Property\ICalendar\DateTime::class, $result);
-        $this->assertEquals('DTSTART', $result->name);
-        $this->assertEquals('20110529', $result->getValue());
+        self::assertInstanceOf(Property\ICalendar\DateTime::class, $result);
+        self::assertEquals('DTSTART', $result->name);
+        self::assertEquals('20110529', $result->getValue());
     }
 
     public function testReadMissingEnd(): void
     {
         $data = "BEGIN:VCALENDAR\r\nPROPNAME:propValue";
         $result = Reader::read($data);
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCALENDAR', $result->name);
-        $this->assertCount(1, $result->children());
-        $this->assertInstanceOf(Property::class, $result->children()[0]);
-        $this->assertEquals('PROPNAME', $result->children()[0]->name);
-        $this->assertEquals('propValue', $result->children()[0]->getValue());
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCALENDAR', $result->name);
+        self::assertCount(1, $result->children());
+        self::assertInstanceOf(Property::class, $result->children()[0]);
+        self::assertEquals('PROPNAME', $result->children()[0]->name);
+        self::assertEquals('propValue', $result->children()[0]->getValue());
     }
 
     public function testReadPropertyInComponent(): void
@@ -136,12 +136,12 @@ public function testReadPropertyInComponent(): void
 
         $result = Reader::read(implode("\r\n", $data));
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCALENDAR', $result->name);
-        $this->assertCount(1, $result->children());
-        $this->assertInstanceOf(Property::class, $result->children()[0]);
-        $this->assertEquals('PROPNAME', $result->children()[0]->name);
-        $this->assertEquals('propValue', $result->children()[0]->getValue());
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCALENDAR', $result->name);
+        self::assertCount(1, $result->children());
+        self::assertInstanceOf(Property::class, $result->children()[0]);
+        self::assertEquals('PROPNAME', $result->children()[0]->name);
+        self::assertEquals('propValue', $result->children()[0]->getValue());
     }
 
     public function testReadNestedComponent(): void
@@ -157,14 +157,14 @@ public function testReadNestedComponent(): void
 
         $result = Reader::read(implode("\r\n", $data));
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCALENDAR', $result->name);
-        $this->assertCount(1, $result->children());
-        $this->assertInstanceOf(Component::class, $result->children()[0]);
-        $this->assertEquals('VTIMEZONE', $result->children()[0]->name);
-        $this->assertCount(1, $result->children()[0]->children());
-        $this->assertInstanceOf(Component::class, $result->children()[0]->children()[0]);
-        $this->assertEquals('DAYLIGHT', $result->children()[0]->children()[0]->name);
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCALENDAR', $result->name);
+        self::assertCount(1, $result->children());
+        self::assertInstanceOf(Component::class, $result->children()[0]);
+        self::assertEquals('VTIMEZONE', $result->children()[0]->name);
+        self::assertCount(1, $result->children()[0]->children());
+        self::assertInstanceOf(Component::class, $result->children()[0]->children()[0]);
+        self::assertEquals('DAYLIGHT', $result->children()[0]->children()[0]->name);
     }
 
     public function testReadPropertyParameter(): void
@@ -174,12 +174,12 @@ public function testReadPropertyParameter(): void
 
         $result = $result->PROPNAME;
 
-        $this->assertInstanceOf(Property::class, $result);
-        $this->assertEquals('PROPNAME', $result->name);
-        $this->assertEquals('propValue', $result->getValue());
-        $this->assertCount(1, $result->parameters());
-        $this->assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
-        $this->assertEquals('paramvalue', $result->parameters['PARAMNAME']->getValue());
+        self::assertInstanceOf(Property::class, $result);
+        self::assertEquals('PROPNAME', $result->name);
+        self::assertEquals('propValue', $result->getValue());
+        self::assertCount(1, $result->parameters());
+        self::assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
+        self::assertEquals('paramvalue', $result->parameters['PARAMNAME']->getValue());
     }
 
     public function testReadPropertyRepeatingParameter(): void
@@ -189,13 +189,13 @@ public function testReadPropertyRepeatingParameter(): void
 
         $result = $result->PROPNAME;
 
-        $this->assertInstanceOf(Property::class, $result);
-        $this->assertEquals('PROPNAME', $result->name);
-        $this->assertEquals('propValue', $result->getValue());
-        $this->assertCount(1, $result->parameters());
-        $this->assertEquals('N', $result->parameters['N']->name);
-        $this->assertEquals('1,2,3,4,5,6,7,8,9,10,"11"', $result->parameters['N']->getValue());
-        $this->assertEquals([1, 2, 3, 4, 5, 6, '7,8', 9, 10, '"11"'], $result->parameters['N']->getParts());
+        self::assertInstanceOf(Property::class, $result);
+        self::assertEquals('PROPNAME', $result->name);
+        self::assertEquals('propValue', $result->getValue());
+        self::assertCount(1, $result->parameters());
+        self::assertEquals('N', $result->parameters['N']->name);
+        self::assertEquals('1,2,3,4,5,6,7,8,9,10,"11"', $result->parameters['N']->getValue());
+        self::assertEquals([1, 2, 3, 4, 5, 6, '7,8', 9, 10, '"11"'], $result->parameters['N']->getParts());
     }
 
     public function testReadPropertyRepeatingNamelessGuessedParameter(): void
@@ -205,13 +205,13 @@ public function testReadPropertyRepeatingNamelessGuessedParameter(): void
 
         $result = $result->PROPNAME;
 
-        $this->assertInstanceOf(Property::class, $result);
-        $this->assertEquals('PROPNAME', $result->name);
-        $this->assertEquals('propValue', $result->getValue());
-        $this->assertCount(1, $result->parameters());
-        $this->assertEquals('TYPE', $result->parameters['TYPE']->name);
-        $this->assertEquals('WORK,VOICE,PREF', $result->parameters['TYPE']->getValue());
-        $this->assertEquals(['WORK', 'VOICE', 'PREF'], $result->parameters['TYPE']->getParts());
+        self::assertInstanceOf(Property::class, $result);
+        self::assertEquals('PROPNAME', $result->name);
+        self::assertEquals('propValue', $result->getValue());
+        self::assertCount(1, $result->parameters());
+        self::assertEquals('TYPE', $result->parameters['TYPE']->name);
+        self::assertEquals('WORK,VOICE,PREF', $result->parameters['TYPE']->getValue());
+        self::assertEquals(['WORK', 'VOICE', 'PREF'], $result->parameters['TYPE']->getParts());
     }
 
     public function testReadPropertyNoName(): void
@@ -221,13 +221,13 @@ public function testReadPropertyNoName(): void
 
         $result = $result->PROPNAME;
 
-        $this->assertInstanceOf(Property::class, $result);
-        $this->assertEquals('PROPNAME', $result->name);
-        $this->assertEquals('propValue', $result->getValue());
-        $this->assertCount(1, $result->parameters());
-        $this->assertEquals('TYPE', $result->parameters['TYPE']->name);
-        $this->assertTrue($result->parameters['TYPE']->noName);
-        $this->assertEquals('PRODIGY', $result->parameters['TYPE']);
+        self::assertInstanceOf(Property::class, $result);
+        self::assertEquals('PROPNAME', $result->name);
+        self::assertEquals('propValue', $result->getValue());
+        self::assertCount(1, $result->parameters());
+        self::assertEquals('TYPE', $result->parameters['TYPE']->name);
+        self::assertTrue($result->parameters['TYPE']->noName);
+        self::assertEquals('PRODIGY', $result->parameters['TYPE']);
     }
 
     public function testReadPropertyParameterExtraColon(): void
@@ -237,12 +237,12 @@ public function testReadPropertyParameterExtraColon(): void
 
         $result = $result->PROPNAME;
 
-        $this->assertInstanceOf(Property::class, $result);
-        $this->assertEquals('PROPNAME', $result->name);
-        $this->assertEquals('propValue:anotherrandomstring', $result->getValue());
-        $this->assertCount(1, $result->parameters());
-        $this->assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
-        $this->assertEquals('paramvalue', $result->parameters['PARAMNAME']->getValue());
+        self::assertInstanceOf(Property::class, $result);
+        self::assertEquals('PROPNAME', $result->name);
+        self::assertEquals('propValue:anotherrandomstring', $result->getValue());
+        self::assertCount(1, $result->parameters());
+        self::assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
+        self::assertEquals('paramvalue', $result->parameters['PARAMNAME']->getValue());
     }
 
     public function testReadProperty2Parameters(): void
@@ -252,14 +252,14 @@ public function testReadProperty2Parameters(): void
 
         $result = $result->PROPNAME;
 
-        $this->assertInstanceOf(Property::class, $result);
-        $this->assertEquals('PROPNAME', $result->name);
-        $this->assertEquals('propValue', $result->getValue());
-        $this->assertCount(2, $result->parameters());
-        $this->assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
-        $this->assertEquals('paramvalue', $result->parameters['PARAMNAME']->getValue());
-        $this->assertEquals('PARAMNAME2', $result->parameters['PARAMNAME2']->name);
-        $this->assertEquals('paramvalue2', $result->parameters['PARAMNAME2']->getValue());
+        self::assertInstanceOf(Property::class, $result);
+        self::assertEquals('PROPNAME', $result->name);
+        self::assertEquals('propValue', $result->getValue());
+        self::assertCount(2, $result->parameters());
+        self::assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
+        self::assertEquals('paramvalue', $result->parameters['PARAMNAME']->getValue());
+        self::assertEquals('PARAMNAME2', $result->parameters['PARAMNAME2']->name);
+        self::assertEquals('paramvalue2', $result->parameters['PARAMNAME2']->getValue());
     }
 
     public function testReadPropertyParameterQuoted(): void
@@ -269,12 +269,12 @@ public function testReadPropertyParameterQuoted(): void
 
         $result = $result->PROPNAME;
 
-        $this->assertInstanceOf(Property::class, $result);
-        $this->assertEquals('PROPNAME', $result->name);
-        $this->assertEquals('propValue', $result->getValue());
-        $this->assertCount(1, $result->parameters());
-        $this->assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
-        $this->assertEquals('paramvalue', $result->parameters['PARAMNAME']->getValue());
+        self::assertInstanceOf(Property::class, $result);
+        self::assertEquals('PROPNAME', $result->name);
+        self::assertEquals('propValue', $result->getValue());
+        self::assertCount(1, $result->parameters());
+        self::assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
+        self::assertEquals('paramvalue', $result->parameters['PARAMNAME']->getValue());
     }
 
     public function testReadPropertyParameterNewLines(): void
@@ -284,13 +284,13 @@ public function testReadPropertyParameterNewLines(): void
 
         $result = $result->PROPNAME;
 
-        $this->assertInstanceOf(Property::class, $result);
-        $this->assertEquals('PROPNAME', $result->name);
-        $this->assertEquals('propValue', $result->getValue());
+        self::assertInstanceOf(Property::class, $result);
+        self::assertEquals('PROPNAME', $result->name);
+        self::assertEquals('propValue', $result->getValue());
 
-        $this->assertCount(1, $result->parameters());
-        $this->assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
-        $this->assertEquals("paramvalue1\nvalue2^nvalue3", $result->parameters['PARAMNAME']->getValue());
+        self::assertCount(1, $result->parameters());
+        self::assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
+        self::assertEquals("paramvalue1\nvalue2^nvalue3", $result->parameters['PARAMNAME']->getValue());
     }
 
     public function testReadPropertyParameterQuotedColon(): void
@@ -299,12 +299,12 @@ public function testReadPropertyParameterQuotedColon(): void
         $result = Reader::read($data);
         $result = $result->PROPNAME;
 
-        $this->assertInstanceOf(Property::class, $result);
-        $this->assertEquals('PROPNAME', $result->name);
-        $this->assertEquals('propValue', $result->getValue());
-        $this->assertCount(1, $result->parameters());
-        $this->assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
-        $this->assertEquals('param:value', $result->parameters['PARAMNAME']->getValue());
+        self::assertInstanceOf(Property::class, $result);
+        self::assertEquals('PROPNAME', $result->name);
+        self::assertEquals('propValue', $result->getValue());
+        self::assertCount(1, $result->parameters());
+        self::assertEquals('PARAMNAME', $result->parameters['PARAMNAME']->name);
+        self::assertEquals('param:value', $result->parameters['PARAMNAME']->getValue());
     }
 
     public function testReadForgiving(): void
@@ -322,7 +322,7 @@ public function testReadForgiving(): void
             $caught = true;
         }
 
-        $this->assertEquals(true, $caught);
+        self::assertEquals(true, $caught);
 
         $result = Reader::read(implode("\r\n", $data), Reader::OPTION_FORGIVING);
 
@@ -333,7 +333,7 @@ public function testReadForgiving(): void
             '',
         ]);
 
-        $this->assertEquals($expected, $result->serialize());
+        self::assertEquals($expected, $result->serialize());
     }
 
     public function testReadWithInvalidLine(): void
@@ -352,7 +352,7 @@ public function testReadWithInvalidLine(): void
             $caught = true;
         }
 
-        $this->assertEquals(true, $caught);
+        self::assertEquals(true, $caught);
 
         $result = Reader::read(implode("\r\n", $data), Reader::OPTION_IGNORE_INVALID_LINES);
 
@@ -363,7 +363,7 @@ public function testReadWithInvalidLine(): void
             '',
         ]);
 
-        $this->assertEquals($expected, $result->serialize());
+        self::assertEquals($expected, $result->serialize());
     }
 
     /**
@@ -413,9 +413,9 @@ public function testReadBOM(): void
         $data = chr(0xEF).chr(0xBB).chr(0xBF)."BEGIN:VCALENDAR\r\nEND:VCALENDAR";
         $result = Reader::read($data);
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCALENDAR', $result->name);
-        $this->assertCount(0, $result->children());
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCALENDAR', $result->name);
+        self::assertCount(0, $result->children());
     }
 
     public function testReadXMLComponent(): void
@@ -430,9 +430,9 @@ public function testReadXMLComponent(): void
 
         $result = Reader::readXML($data);
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCALENDAR', $result->name);
-        $this->assertCount(0, $result->children());
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCALENDAR', $result->name);
+        self::assertCount(0, $result->children());
     }
 
     public function testReadXMLStream(): void
@@ -451,8 +451,8 @@ public function testReadXMLStream(): void
 
         $result = Reader::readXML($stream);
 
-        $this->assertInstanceOf(Component::class, $result);
-        $this->assertEquals('VCALENDAR', $result->name);
-        $this->assertCount(0, $result->children());
+        self::assertInstanceOf(Component::class, $result);
+        self::assertEquals('VCALENDAR', $result->name);
+        self::assertCount(0, $result->children());
     }
 }
diff --git a/tests/VObject/Recur/EventIterator/ByMonthInDailyTest.php b/tests/VObject/Recur/EventIterator/ByMonthInDailyTest.php
index 3e1fb199..9b6b20df 100644
--- a/tests/VObject/Recur/EventIterator/ByMonthInDailyTest.php
+++ b/tests/VObject/Recur/EventIterator/ByMonthInDailyTest.php
@@ -36,7 +36,7 @@ public function testExpand(): void
 ICS;
 
         $vcal = Reader::read($ics);
-        $this->assertInstanceOf(VCalendar::class, $vcal);
+        self::assertInstanceOf(VCalendar::class, $vcal);
 
         $vcal = $vcal->expand(new \DateTime('2013-09-28'), new \DateTime('2014-09-11'));
 
@@ -54,6 +54,6 @@ public function testExpand(): void
             '20140907T160000Z',
         ];
 
-        $this->assertEquals($expectedDates, $dates, 'Recursed dates are restricted by month');
+        self::assertEquals($expectedDates, $dates, 'Recursed dates are restricted by month');
     }
 }
diff --git a/tests/VObject/Recur/EventIterator/BySetPosHangTest.php b/tests/VObject/Recur/EventIterator/BySetPosHangTest.php
index 04055c72..f5132e1d 100644
--- a/tests/VObject/Recur/EventIterator/BySetPosHangTest.php
+++ b/tests/VObject/Recur/EventIterator/BySetPosHangTest.php
@@ -31,7 +31,7 @@ public function testExpand(): void
 ICS;
 
         $vcal = Reader::read($ics);
-        $this->assertInstanceOf(VCalendar::class, $vcal);
+        self::assertInstanceOf(VCalendar::class, $vcal);
 
         $vcal = $vcal->expand(new \DateTime('2015-01-01'), new \DateTime('2016-01-01'));
 
@@ -56,6 +56,6 @@ public function testExpand(): void
             '20151224T160000Z',
         ];
 
-        $this->assertEquals($expectedDates, $dates);
+        self::assertEquals($expectedDates, $dates);
     }
 }
diff --git a/tests/VObject/Recur/EventIterator/ExpandFloatingTimesTest.php b/tests/VObject/Recur/EventIterator/ExpandFloatingTimesTest.php
index e764a76c..a7a39d1d 100644
--- a/tests/VObject/Recur/EventIterator/ExpandFloatingTimesTest.php
+++ b/tests/VObject/Recur/EventIterator/ExpandFloatingTimesTest.php
@@ -25,7 +25,7 @@ public function testExpand(): void
 ICS;
 
         $vcal = Reader::read($input);
-        $this->assertInstanceOf(VCalendar::class, $vcal);
+        self::assertInstanceOf(VCalendar::class, $vcal);
 
         $vcal = $vcal->expand(new \DateTime('2015-01-01'), new \DateTime('2015-01-31'));
         $output = <<<ICS
@@ -58,7 +58,7 @@ public function testExpand(): void
 END:VCALENDAR
 
 ICS;
-        $this->assertVObjectEqualsVObject($output, $vcal);
+        self::assertVObjectEqualsVObject($output, $vcal);
     }
 
     public function testExpandWithReferenceTimezone(): void
@@ -76,7 +76,7 @@ public function testExpandWithReferenceTimezone(): void
 ICS;
 
         $vcal = Reader::read($input);
-        $this->assertInstanceOf(VCalendar::class, $vcal);
+        self::assertInstanceOf(VCalendar::class, $vcal);
 
         $vcal = $vcal->expand(
             new \DateTime('2015-01-01'),
@@ -114,6 +114,6 @@ public function testExpandWithReferenceTimezone(): void
 END:VCALENDAR
 
 ICS;
-        $this->assertVObjectEqualsVObject($output, $vcal);
+        self::assertVObjectEqualsVObject($output, $vcal);
     }
 }
diff --git a/tests/VObject/Recur/EventIterator/FifthTuesdayProblemTest.php b/tests/VObject/Recur/EventIterator/FifthTuesdayProblemTest.php
index 018af065..8391a232 100644
--- a/tests/VObject/Recur/EventIterator/FifthTuesdayProblemTest.php
+++ b/tests/VObject/Recur/EventIterator/FifthTuesdayProblemTest.php
@@ -48,6 +48,6 @@ public function testGetDTEnd(): void
         // If we got here, it means we were successful. The bug that was in the
         // system before would fail on the 5th tuesday of the month, if the 5th
         // tuesday did not exist.
-        $this->assertTrue(true);
+        self::assertTrue(true);
     }
 }
diff --git a/tests/VObject/Recur/EventIterator/HandleRDateExpandTest.php b/tests/VObject/Recur/EventIterator/HandleRDateExpandTest.php
index e3ec52e2..4f434dd2 100644
--- a/tests/VObject/Recur/EventIterator/HandleRDateExpandTest.php
+++ b/tests/VObject/Recur/EventIterator/HandleRDateExpandTest.php
@@ -34,13 +34,13 @@ public function testExpand(): void
 ICS;
 
         $vcal = Reader::read($input);
-        $this->assertInstanceOf(VCalendar::class, $vcal);
+        self::assertInstanceOf(VCalendar::class, $vcal);
 
         $vcal = $vcal->expand(new \DateTime('2015-01-01'), new \DateTime('2015-12-01'));
 
         $result = iterator_to_array($vcal->VEVENT);
 
-        $this->assertCount(5, $result);
+        self::assertCount(5, $result);
 
         $utc = new \DateTimeZone('UTC');
         $expected = [
@@ -52,6 +52,6 @@ public function testExpand(): void
         ];
 
         $result = array_map(function ($ev) {return $ev->DTSTART->getDateTime(); }, $result);
-        $this->assertEquals($expected, $result);
+        self::assertEquals($expected, $result);
     }
 }
diff --git a/tests/VObject/Recur/EventIterator/IncorrectExpandTest.php b/tests/VObject/Recur/EventIterator/IncorrectExpandTest.php
index 3e5dad08..b0ecbbdb 100644
--- a/tests/VObject/Recur/EventIterator/IncorrectExpandTest.php
+++ b/tests/VObject/Recur/EventIterator/IncorrectExpandTest.php
@@ -34,7 +34,7 @@ public function testExpand(): void
 ICS;
 
         $vcal = Reader::read($input);
-        $this->assertInstanceOf(VCalendar::class, $vcal);
+        self::assertInstanceOf(VCalendar::class, $vcal);
 
         $vcal = $vcal->expand(new \DateTime('2011-01-01'), new \DateTime('2014-01-01'));
 
@@ -56,6 +56,6 @@ public function testExpand(): void
 END:VCALENDAR
 
 ICS;
-        $this->assertVObjectEqualsVObject($output, $vcal);
+        self::assertVObjectEqualsVObject($output, $vcal);
     }
 }
diff --git a/tests/VObject/Recur/EventIterator/InfiniteLoopProblemTest.php b/tests/VObject/Recur/EventIterator/InfiniteLoopProblemTest.php
index dafbf70f..591140e6 100644
--- a/tests/VObject/Recur/EventIterator/InfiniteLoopProblemTest.php
+++ b/tests/VObject/Recur/EventIterator/InfiniteLoopProblemTest.php
@@ -27,7 +27,7 @@ public function testFastForwardTooFar(): void
         $ev->DTSTART = '20090420T180000Z';
         $ev->RRULE = 'FREQ=WEEKLY;BYDAY=MO;UNTIL=20090704T205959Z;INTERVAL=1';
 
-        $this->assertFalse($ev->isInTimeRange(new \DateTimeImmutable('2012-01-01 12:00:00'), new \DateTimeImmutable('3000-01-01 00:00:00')));
+        self::assertFalse($ev->isInTimeRange(new \DateTimeImmutable('2012-01-01 12:00:00'), new \DateTimeImmutable('3000-01-01 00:00:00')));
     }
 
     /**
@@ -64,7 +64,7 @@ public function testYearlyByMonthLoop(): void
             $it->next();
         }
 
-        $this->assertEquals(
+        self::assertEquals(
             [new \DateTimeImmutable('2012-02-01 15:45:00', new \DateTimeZone('Europe/Berlin'))],
             $collect
         );
diff --git a/tests/VObject/Recur/EventIterator/Issue26Test.php b/tests/VObject/Recur/EventIterator/Issue26Test.php
index 3be5edf0..06cd6388 100644
--- a/tests/VObject/Recur/EventIterator/Issue26Test.php
+++ b/tests/VObject/Recur/EventIterator/Issue26Test.php
@@ -26,7 +26,7 @@ public function testExpand(): void
 ICS;
 
         $vcal = Reader::read($input);
-        $this->assertInstanceOf(VCalendar::class, $vcal);
+        self::assertInstanceOf(VCalendar::class, $vcal);
 
         new EventIterator($vcal, 'bae5d57a98');
     }
diff --git a/tests/VObject/Recur/EventIterator/Issue48Test.php b/tests/VObject/Recur/EventIterator/Issue48Test.php
index 84a8e901..d2b80936 100644
--- a/tests/VObject/Recur/EventIterator/Issue48Test.php
+++ b/tests/VObject/Recur/EventIterator/Issue48Test.php
@@ -29,7 +29,7 @@ public function testExpand(): void
 ICS;
 
         $vcal = Reader::read($input);
-        $this->assertInstanceOf(VCalendar::class, $vcal);
+        self::assertInstanceOf(VCalendar::class, $vcal);
 
         $it = new EventIterator($vcal, 'foo');
 
@@ -43,6 +43,6 @@ public function testExpand(): void
             new \DateTimeImmutable('2013-07-13 11:00:00', $tz),
         ];
 
-        $this->assertEquals($expected, $result);
+        self::assertEquals($expected, $result);
     }
 }
diff --git a/tests/VObject/Recur/EventIterator/Issue50Test.php b/tests/VObject/Recur/EventIterator/Issue50Test.php
index 4e1fd33f..ec05de1b 100644
--- a/tests/VObject/Recur/EventIterator/Issue50Test.php
+++ b/tests/VObject/Recur/EventIterator/Issue50Test.php
@@ -103,7 +103,7 @@ public function testExpand(): void
 ICS;
 
         $vcal = Reader::read($input);
-        $this->assertInstanceOf(VCalendar::class, $vcal);
+        self::assertInstanceOf(VCalendar::class, $vcal);
 
         $it = new EventIterator($vcal, '1aef0b27-3d92-4581-829a-11999dd36724');
 
@@ -114,7 +114,7 @@ public function testExpand(): void
 
         $tz = new \DateTimeZone('Europe/Brussels');
 
-        $this->assertEquals([
+        self::assertEquals([
             new \DateTimeImmutable('2013-07-15 09:00:00', $tz),
             new \DateTimeImmutable('2013-07-16 07:00:00', $tz),
             new \DateTimeImmutable('2013-07-17 07:00:00', $tz),
diff --git a/tests/VObject/Recur/EventIterator/MainTest.php b/tests/VObject/Recur/EventIterator/MainTest.php
index af4a8be7..bce4bcd9 100644
--- a/tests/VObject/Recur/EventIterator/MainTest.php
+++ b/tests/VObject/Recur/EventIterator/MainTest.php
@@ -35,7 +35,7 @@ public function testValues(): void
 
         $it = new EventIterator($vcal, (string) $ev->UID);
 
-        $this->assertTrue($it->isInfinite());
+        self::assertTrue($it->isInfinite());
     }
 
     /**
@@ -122,7 +122,7 @@ public function testHourly(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-10-07 12:00:00', $tz),
                 new \DateTimeImmutable('2011-10-07 15:00:00', $tz),
@@ -181,7 +181,7 @@ public function testDaily(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-10-07', $tz),
                 new \DateTimeImmutable('2011-10-10', $tz),
@@ -227,7 +227,7 @@ public function testNoRRULE(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-10-07', $tz),
             ],
@@ -268,7 +268,7 @@ public function testDailyByDayByHour(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-10-08 06:00:00', $tz),
                 new \DateTimeImmutable('2011-10-08 07:00:00', $tz),
@@ -320,7 +320,7 @@ public function testDailyByHour(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2012-10-11 12:00:00', $tz),
                 new \DateTimeImmutable('2012-10-11 13:00:00', $tz),
@@ -372,7 +372,7 @@ public function testDailyByDay(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-10-07', $tz),
                 new \DateTimeImmutable('2011-10-11', $tz),
@@ -424,7 +424,7 @@ public function testWeekly(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-10-07', $tz),
                 new \DateTimeImmutable('2011-10-21', $tz),
@@ -474,7 +474,7 @@ public function testWeeklyByDayByHour(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-10-07 08:00:00', $tz),
                 new \DateTimeImmutable('2011-10-07 09:00:00', $tz),
@@ -529,7 +529,7 @@ public function testWeeklyByDaySpecificHour(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-10-07 18:00:00', $tz),
                 new \DateTimeImmutable('2011-10-18 18:00:00', $tz),
@@ -581,7 +581,7 @@ public function testWeeklyByDay(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-10-07', $tz),
                 new \DateTimeImmutable('2011-10-18', $tz),
@@ -632,7 +632,7 @@ public function testMonthly(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-12-05', $tz),
                 new \DateTimeImmutable('2012-03-05', $tz),
@@ -676,7 +676,7 @@ public function testMonthlyEndOfMonth(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-12-31', $tz),
                 new \DateTimeImmutable('2012-08-31', $tz),
@@ -734,7 +734,7 @@ public function testMonthlyByMonthDay(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-01-01', $tz),
                 new \DateTimeImmutable('2011-01-25', $tz),
@@ -787,7 +787,7 @@ public function testMonthlyByDay(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-01-03', $tz),
                 new \DateTimeImmutable('2011-01-05', $tz),
@@ -842,7 +842,7 @@ public function testMonthlyByDayByMonthDay(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-08-01', $tz),
                 new \DateTimeImmutable('2012-10-01', $tz),
@@ -891,7 +891,7 @@ public function testMonthlyByDayBySetPos(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-01-03', $tz),
                 new \DateTimeImmutable('2011-01-31', $tz),
@@ -945,7 +945,7 @@ public function testYearly(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-01-01', $tz),
                 new \DateTimeImmutable('2014-01-01', $tz),
@@ -994,7 +994,7 @@ public function testYearlyLeapYear(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2012-02-29', $tz),
                 new \DateTimeImmutable('2016-02-29', $tz),
@@ -1036,7 +1036,7 @@ public function testYearlyByMonth(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-04-07', $tz),
                 new \DateTimeImmutable('2011-10-07', $tz),
@@ -1083,7 +1083,7 @@ public function testYearlyByMonthByDay(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-04-04', $tz),
                 new \DateTimeImmutable('2011-04-24', $tz),
@@ -1133,7 +1133,7 @@ public function testFastForward(): void
             $it->next();
         }
 
-        $this->assertEquals([], $result);
+        self::assertEquals([], $result);
     }
 
     /**
@@ -1161,7 +1161,7 @@ public function testFastForwardAllDayEventThatStopAtTheStartTime(): void
 
         $it->fastForward(new \DateTimeImmutable('2011-04-05T000000', new \DateTimeZone('UTC')));
 
-        $this->assertEquals(new \DateTimeImmutable('2011-04-06'), $it->getDTStart());
+        self::assertEquals(new \DateTimeImmutable('2011-04-06'), $it->getDTStart());
     }
 
     /**
@@ -1212,7 +1212,7 @@ public function testComplexExclusions(): void
             }
         }
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-01-01 13:50:20', $tz),
                 new \DateTimeImmutable('2013-01-01 13:50:20', $tz),
@@ -1270,7 +1270,7 @@ public function testOverriddenEvent(): void
         }
 
         $tz = new \DateTimeZone('UTC');
-        $this->assertEquals([
+        self::assertEquals([
             new \DateTimeImmutable('2012-01-07 12:00:00', $tz),
             new \DateTimeImmutable('2012-01-08 12:00:00', $tz),
             new \DateTimeImmutable('2012-01-09 12:00:00', $tz),
@@ -1283,7 +1283,7 @@ public function testOverriddenEvent(): void
             new \DateTimeImmutable('2012-01-16 12:00:00', $tz),
         ], $dates);
 
-        $this->assertEquals([
+        self::assertEquals([
             'baseEvent',
             'baseEvent',
             'baseEvent',
@@ -1332,13 +1332,13 @@ public function testOverriddenEvent2(): void
         }
 
         $tz = new \DateTimeZone('UTC');
-        $this->assertEquals([
+        self::assertEquals([
             new \DateTimeImmutable('2012-01-12 12:00:00', $tz),
             new \DateTimeImmutable('2012-01-13 12:00:00', $tz),
             new \DateTimeImmutable('2012-01-26 12:00:00', $tz),
         ], $dates);
 
-        $this->assertEquals([
+        self::assertEquals([
             'baseEvent',
             'Override!',
             'baseEvent',
@@ -1388,8 +1388,8 @@ public function testOverriddenEventNoValuesExpected(): void
             $it->next();
         }
 
-        $this->assertEquals([], $dates);
-        $this->assertEquals([], $summaries);
+        self::assertEquals([], $dates);
+        self::assertEquals([], $summaries);
     }
 
     /**
@@ -1428,7 +1428,7 @@ public function testRDATE(): void
 
         $tz = new \DateTimeZone('UTC');
 
-        $this->assertEquals(
+        self::assertEquals(
             [
                 new \DateTimeImmutable('2011-10-07', $tz),
                 new \DateTimeImmutable('2014-08-07', $tz),
diff --git a/tests/VObject/Recur/EventIterator/MissingOverriddenTest.php b/tests/VObject/Recur/EventIterator/MissingOverriddenTest.php
index f848412e..2b1656ec 100644
--- a/tests/VObject/Recur/EventIterator/MissingOverriddenTest.php
+++ b/tests/VObject/Recur/EventIterator/MissingOverriddenTest.php
@@ -33,7 +33,7 @@ public function testExpand(): void
 ICS;
 
         $vcal = Reader::read($input);
-        $this->assertInstanceOf(VCalendar::class, $vcal);
+        self::assertInstanceOf(VCalendar::class, $vcal);
 
         $vcal = $vcal->expand(new \DateTime('2011-01-01'), new \DateTime('2015-01-01'));
 
@@ -56,6 +56,6 @@ public function testExpand(): void
 END:VEVENT
 END:VCALENDAR
 ICS;
-        $this->assertVObjectEqualsVObject($output, $vcal);
+        self::assertVObjectEqualsVObject($output, $vcal);
     }
 }
diff --git a/tests/VObject/Recur/EventIterator/NoInstancesTest.php b/tests/VObject/Recur/EventIterator/NoInstancesTest.php
index 329543b7..1eff5f94 100644
--- a/tests/VObject/Recur/EventIterator/NoInstancesTest.php
+++ b/tests/VObject/Recur/EventIterator/NoInstancesTest.php
@@ -32,7 +32,7 @@ public function testRecurrence(): void
 ICS;
 
         $vcal = Reader::read($input);
-        $this->assertInstanceOf(VCalendar::class, $vcal);
+        self::assertInstanceOf(VCalendar::class, $vcal);
 
         new EventIterator($vcal, 'foo');
     }
diff --git a/tests/VObject/Recur/EventIterator/OverrideDurationTest.php b/tests/VObject/Recur/EventIterator/OverrideDurationTest.php
index da492abb..8162a614 100644
--- a/tests/VObject/Recur/EventIterator/OverrideDurationTest.php
+++ b/tests/VObject/Recur/EventIterator/OverrideDurationTest.php
@@ -34,19 +34,19 @@ public function testOverrideDuration(): void
         $vCalendar = Reader::read($ics);
         $eventIterator = new EventIterator($vCalendar->getComponents());
 
-        $this->assertEquals('2021-05-17 09:00:00', $eventIterator->current()->format('Y-m-d H:i:s'), 'recur event start time');
-        $this->assertEquals('2021-05-17 10:00:00', $eventIterator->getDtEnd()->format('Y-m-d H:i:s'), 'recur event end time');
+        self::assertEquals('2021-05-17 09:00:00', $eventIterator->current()->format('Y-m-d H:i:s'), 'recur event start time');
+        self::assertEquals('2021-05-17 10:00:00', $eventIterator->getDtEnd()->format('Y-m-d H:i:s'), 'recur event end time');
 
         $eventIterator->next();
-        $this->assertEquals('2021-05-18 09:00:00', $eventIterator->current()->format('Y-m-d H:i:s'), 'recur event start time');
-        $this->assertEquals('2021-05-18 10:00:00', $eventIterator->getDtEnd()->format('Y-m-d H:i:s'), 'recur event end time');
+        self::assertEquals('2021-05-18 09:00:00', $eventIterator->current()->format('Y-m-d H:i:s'), 'recur event start time');
+        self::assertEquals('2021-05-18 10:00:00', $eventIterator->getDtEnd()->format('Y-m-d H:i:s'), 'recur event end time');
 
         $eventIterator->next();
-        $this->assertEquals('2021-05-19 09:00:00', $eventIterator->current()->format('Y-m-d H:i:s'), 'overridden event start time');
-        $this->assertEquals('2021-05-19 12:00:00', $eventIterator->getDtEnd()->format('Y-m-d H:i:s'), 'overridden event end time');
+        self::assertEquals('2021-05-19 09:00:00', $eventIterator->current()->format('Y-m-d H:i:s'), 'overridden event start time');
+        self::assertEquals('2021-05-19 12:00:00', $eventIterator->getDtEnd()->format('Y-m-d H:i:s'), 'overridden event end time');
 
         $eventIterator->next();
-        $this->assertEquals('2021-05-20 09:00:00', $eventIterator->current()->format('Y-m-d H:i:s'), 'recur event start time');
-        $this->assertEquals('2021-05-20 10:00:00', $eventIterator->getDtEnd()->format('Y-m-d H:i:s'), 'recur event end time');
+        self::assertEquals('2021-05-20 09:00:00', $eventIterator->current()->format('Y-m-d H:i:s'), 'recur event start time');
+        self::assertEquals('2021-05-20 10:00:00', $eventIterator->getDtEnd()->format('Y-m-d H:i:s'), 'recur event end time');
     }
 }
diff --git a/tests/VObject/Recur/EventIterator/OverrideFirstEventTest.php b/tests/VObject/Recur/EventIterator/OverrideFirstEventTest.php
index 53914a65..15c92f04 100644
--- a/tests/VObject/Recur/EventIterator/OverrideFirstEventTest.php
+++ b/tests/VObject/Recur/EventIterator/OverrideFirstEventTest.php
@@ -68,7 +68,7 @@ public function testOverrideFirstEvent(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $vcal
         );
@@ -110,7 +110,7 @@ public function testRemoveFirstEvent(): void
 END:VCALENDAR
 ICS;
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $expected,
             $vcal
         );
diff --git a/tests/VObject/Recur/EventIterator/SameDateForRecurringEventsTest.php b/tests/VObject/Recur/EventIterator/SameDateForRecurringEventsTest.php
index 64b477c2..ef3ed9b1 100644
--- a/tests/VObject/Recur/EventIterator/SameDateForRecurringEventsTest.php
+++ b/tests/VObject/Recur/EventIterator/SameDateForRecurringEventsTest.php
@@ -51,6 +51,6 @@ public function testAllEventsArePresentInIterator(): void
         $vCalendar = Reader::read($ics);
         $eventIterator = new EventIterator($vCalendar->getComponents());
 
-        $this->assertEquals(4, iterator_count($eventIterator), 'in ICS 4 events');
+        self::assertEquals(4, iterator_count($eventIterator), 'in ICS 4 events');
     }
 }
diff --git a/tests/VObject/Recur/RDateIteratorTest.php b/tests/VObject/Recur/RDateIteratorTest.php
index 56c75fb6..5dfe5738 100644
--- a/tests/VObject/Recur/RDateIteratorTest.php
+++ b/tests/VObject/Recur/RDateIteratorTest.php
@@ -17,12 +17,12 @@ public function testSimple(): void
             new \DateTimeImmutable('2014-10-01 00:00:00', $utc),
         ];
 
-        $this->assertEquals(
+        self::assertEquals(
             $expected,
             iterator_to_array($it)
         );
 
-        $this->assertFalse($it->isInfinite());
+        self::assertFalse($it->isInfinite());
     }
 
     public function testTimezone(): void
@@ -36,12 +36,12 @@ public function testTimezone(): void
             new \DateTimeImmutable('2014-10-01 00:00:00', $tz),
         ];
 
-        $this->assertEquals(
+        self::assertEquals(
             $expected,
             iterator_to_array($it)
         );
 
-        $this->assertFalse($it->isInfinite());
+        self::assertFalse($it->isInfinite());
     }
 
     public function testFastForward(): void
@@ -62,11 +62,11 @@ public function testFastForward(): void
             new \DateTimeImmutable('2014-10-01 00:00:00', $utc),
         ];
 
-        $this->assertEquals(
+        self::assertEquals(
             $expected,
             $result
         );
 
-        $this->assertFalse($it->isInfinite());
+        self::assertFalse($it->isInfinite());
     }
 }
diff --git a/tests/VObject/Recur/RRuleIteratorTest.php b/tests/VObject/Recur/RRuleIteratorTest.php
index 04b636e7..2814df66 100644
--- a/tests/VObject/Recur/RRuleIteratorTest.php
+++ b/tests/VObject/Recur/RRuleIteratorTest.php
@@ -573,7 +573,7 @@ public function testYearlyByYearDayImmutable(): void
         $parser->next();
 
         $item = $parser->current();
-        $this->assertEquals('2013-07-09 03:07:00', $item->format('Y-m-d H:i:s'));
+        self::assertEquals('2013-07-09 03:07:00', $item->format('Y-m-d H:i:s'));
     }
 
     public function testYearlyByYearDayMultiple(): void
@@ -1051,22 +1051,22 @@ public function testIteratorFunctions(): void
     {
         $parser = new RRuleIterator('FREQ=DAILY', new \DateTime('2014-08-02 00:00:13'));
         $parser->next();
-        $this->assertEquals(
+        self::assertEquals(
             new \DateTime('2014-08-03 00:00:13'),
             $parser->current()
         );
-        $this->assertEquals(
+        self::assertEquals(
             1,
             $parser->key()
         );
 
         $parser->rewind();
 
-        $this->assertEquals(
+        self::assertEquals(
             new \DateTime('2014-08-02 00:00:13'),
             $parser->current()
         );
-        $this->assertEquals(
+        self::assertEquals(
             0,
             $parser->key()
         );
@@ -1092,7 +1092,7 @@ public function parse($rule, string $start, array $expected, string $fastForward
             $parser->next();
         }
 
-        $this->assertEquals(
+        self::assertEquals(
             $expected,
             $result
         );
diff --git a/tests/VObject/SlashRTest.php b/tests/VObject/SlashRTest.php
index 81ab87df..8f968d3c 100644
--- a/tests/VObject/SlashRTest.php
+++ b/tests/VObject/SlashRTest.php
@@ -14,6 +14,6 @@ public function testEncode(): void
     {
         $vcal = new Component\VCalendar();
         $prop = $vcal->add('test', "abc\r\ndef");
-        $this->assertEquals("TEST:abc\\ndef\r\n", $prop->serialize());
+        self::assertEquals("TEST:abc\\ndef\r\n", $prop->serialize());
     }
 }
diff --git a/tests/VObject/Splitter/ICalendarTest.php b/tests/VObject/Splitter/ICalendarTest.php
index fbef506c..f5428e93 100644
--- a/tests/VObject/Splitter/ICalendarTest.php
+++ b/tests/VObject/Splitter/ICalendarTest.php
@@ -43,7 +43,7 @@ public function testICalendarImportValidEvent(): void
         while ($object = $objects->getNext()) {
             $return .= $object->serialize();
         }
-        $this->assertEquals([], VObject\Reader::read($return)->validate());
+        self::assertEquals([], VObject\Reader::read($return)->validate());
     }
 
     public function testICalendarImportWrongType(): void
@@ -80,7 +80,7 @@ public function testICalendarImportEndOfData(): void
         while ($object = $objects->getNext()) {
             $return .= $object->serialize();
         }
-        $this->assertNull($objects->getNext());
+        self::assertNull($objects->getNext());
     }
 
     public function testICalendarImportInvalidEvent(): void
@@ -136,10 +136,10 @@ public function testICalendarImportMultipleValidEvents(): void
 
             $return .= $object->serialize();
             $expected = str_replace("\n", "\r\n", $expected);
-            $this->assertEquals($expected, $object->serialize());
+            self::assertEquals($expected, $object->serialize());
             ++$i;
         }
-        $this->assertEquals([], VObject\Reader::read($return)->validate());
+        self::assertEquals([], VObject\Reader::read($return)->validate());
     }
 
     public function testICalendarImportEventWithoutUID(): void
@@ -174,7 +174,7 @@ function ($item) { return $item['message']; },
             );
             $this->fail('Validation errors: '.implode("\n", $messages));
         } else {
-            $this->assertEquals([], $messages);
+            self::assertEquals([], $messages);
         }
     }
 
@@ -269,12 +269,12 @@ public function testICalendarImportMultipleVTIMEZONESAndMultipleValidEvents(): v
 EOT;
             $expected = str_replace("\n", "\r\n", $expected);
 
-            $this->assertEquals($expected, $object->serialize());
+            self::assertEquals($expected, $object->serialize());
             $return .= $object->serialize();
             ++$i;
         }
 
-        $this->assertEquals([], VObject\Reader::read($return)->validate());
+        self::assertEquals([], VObject\Reader::read($return)->validate());
     }
 
     public function testICalendarImportWithOutVTIMEZONES(): void
@@ -315,6 +315,6 @@ public function testICalendarImportWithOutVTIMEZONES(): void
         }
 
         $messages = VObject\Reader::read($return)->validate();
-        $this->assertEquals([], $messages);
+        self::assertEquals([], $messages);
     }
 }
diff --git a/tests/VObject/Splitter/VCardTest.php b/tests/VObject/Splitter/VCardTest.php
index 11194d66..7ebb07b9 100644
--- a/tests/VObject/Splitter/VCardTest.php
+++ b/tests/VObject/Splitter/VCardTest.php
@@ -31,7 +31,7 @@ public function testVCardImportValidVCard(): void
         while ($objects->getNext()) {
             ++$count;
         }
-        $this->assertEquals(1, $count);
+        self::assertEquals(1, $count);
     }
 
     public function testVCardImportWrongType(): void
@@ -96,7 +96,7 @@ public function testVCardImportValidVCardsWithCategories(): void
         while ($splitter->getNext()) {
             ++$count;
         }
-        $this->assertEquals(4, $count);
+        self::assertEquals(4, $count);
     }
 
     public function testVCardImportVCardNoComponent(): void
@@ -140,7 +140,7 @@ public function testVCardImportQuotedPrintableOptionForgivingLeading(): void
         while ($splitter->getNext()) {
             ++$count;
         }
-        $this->assertEquals(2, $count);
+        self::assertEquals(2, $count);
     }
 
     public function testVCardImportEndOfData(): void
@@ -155,7 +155,7 @@ public function testVCardImportEndOfData(): void
         $objects = new VCard($tempFile);
         $objects->getNext();
 
-        $this->assertNull($objects->getNext());
+        self::assertNull($objects->getNext());
     }
 
     public function testVCardImportCheckInvalidArgumentException(): void
@@ -190,7 +190,7 @@ public function testVCardImportMultipleValidVCards(): void
         while ($objects->getNext()) {
             ++$count;
         }
-        $this->assertEquals(2, $count);
+        self::assertEquals(2, $count);
     }
 
     public function testImportMultipleSeparatedWithNewLines(): void
@@ -214,7 +214,7 @@ public function testImportMultipleSeparatedWithNewLines(): void
         while ($objects->getNext()) {
             ++$count;
         }
-        $this->assertEquals(2, $count);
+        self::assertEquals(2, $count);
     }
 
     public function testVCardImportVCardWithoutUID(): void
@@ -232,6 +232,6 @@ public function testVCardImportVCardWithoutUID(): void
             ++$count;
         }
 
-        $this->assertEquals(1, $count);
+        self::assertEquals(1, $count);
     }
 }
diff --git a/tests/VObject/StringUtilTest.php b/tests/VObject/StringUtilTest.php
index 4dd8bab7..6d6b9b05 100644
--- a/tests/VObject/StringUtilTest.php
+++ b/tests/VObject/StringUtilTest.php
@@ -10,21 +10,21 @@ public function testNonUTF8(): void
     {
         $string = StringUtil::isUTF8(chr(0xBF));
 
-        $this->assertEquals(false, $string);
+        self::assertEquals(false, $string);
     }
 
     public function testIsUTF8(): void
     {
         $string = StringUtil::isUTF8('I 💚 SabreDAV');
 
-        $this->assertEquals(true, $string);
+        self::assertEquals(true, $string);
     }
 
     public function testUTF8ControlChar(): void
     {
         $string = StringUtil::isUTF8(chr(0x00));
 
-        $this->assertEquals(false, $string);
+        self::assertEquals(false, $string);
     }
 
     public function testConvertToUTF8nonUTF8(): void
@@ -32,20 +32,20 @@ public function testConvertToUTF8nonUTF8(): void
         // 0xBF is an ASCII upside-down question mark
         $string = StringUtil::convertToUTF8(chr(0xBF));
 
-        $this->assertEquals(mb_convert_encoding(chr(0xBF), 'UTF-8', 'ISO-8859-1'), $string);
+        self::assertEquals(mb_convert_encoding(chr(0xBF), 'UTF-8', 'ISO-8859-1'), $string);
     }
 
     public function testConvertToUTF8IsUTF8(): void
     {
         $string = StringUtil::convertToUTF8('I 💚 SabreDAV');
 
-        $this->assertEquals('I 💚 SabreDAV', $string);
+        self::assertEquals('I 💚 SabreDAV', $string);
     }
 
     public function testConvertToUTF8ControlChar(): void
     {
         $string = StringUtil::convertToUTF8(chr(0x00));
 
-        $this->assertEquals('', $string);
+        self::assertEquals('', $string);
     }
 }
diff --git a/tests/VObject/TimeZoneUtilTest.php b/tests/VObject/TimeZoneUtilTest.php
index 2b53fa9d..462218e9 100644
--- a/tests/VObject/TimeZoneUtilTest.php
+++ b/tests/VObject/TimeZoneUtilTest.php
@@ -18,7 +18,7 @@ public function testCorrectTZ(string $timezoneName): void
     {
         try {
             $tz = new \DateTimeZone($timezoneName);
-            $this->assertInstanceOf('DateTimeZone', $tz);
+            self::assertInstanceOf('DateTimeZone', $tz);
         } catch (\Exception $e) {
             if (false !== strpos($e->getMessage(), 'Unknown or bad timezone')) {
                 $this->markTestSkipped($timezoneName.' is not (yet) supported in this PHP version. Update pecl/timezonedb');
@@ -83,7 +83,7 @@ public function testExchangeMap(): void
         $tz = TimeZoneUtil::getTimeZone('foo', Reader::read($vobj));
         $ex = new \DateTimeZone('Europe/Lisbon');
 
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 
     public function testWhetherMicrosoftIsStillInsane(): void
@@ -108,7 +108,7 @@ public function testWhetherMicrosoftIsStillInsane(): void
         $tz = TimeZoneUtil::getTimeZone('(GMT+01.00) Sarajevo/Warsaw/Zagreb', Reader::read($vobj));
         $ex = new \DateTimeZone('Europe/Sarajevo');
 
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 
     public function testUnknownExchangeId(): void
@@ -148,21 +148,21 @@ public function testUnknownExchangeId(): void
 
         $tz = TimeZoneUtil::getTimeZone('foo', Reader::read($vobj));
         $ex = new \DateTimeZone(date_default_timezone_get());
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 
     public function testEmptyTimeZone(): void
     {
         $tz = TimeZoneUtil::getTimeZone('');
         $ex = new \DateTimeZone('UTC');
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 
     public function testWindowsTimeZone(): void
     {
         $tz = TimeZoneUtil::getTimeZone('Eastern Standard Time');
         $ex = new \DateTimeZone('America/New_York');
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 
     /**
@@ -173,7 +173,7 @@ public function testTimeZoneIdentifiers(string $tzid): void
         $tz = TimeZoneUtil::getTimeZone($tzid);
         $ex = new \DateTimeZone($tzid);
 
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 
     /**
@@ -195,7 +195,7 @@ public function testTimeZoneBCIdentifiers(string $tzid): void
         $tz = TimeZoneUtil::getTimeZone($tzid);
         $ex = new \DateTimeZone($tzid);
 
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 
     public function getPHPTimeZoneIdentifiers(): array
@@ -229,7 +229,7 @@ public function testTimezoneOffset(): void
         } else {
             $ex = new \DateTimeZone('Etc/GMT-4');
         }
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 
     public function testTimezoneFail(): void
@@ -273,7 +273,7 @@ public function testFallBack(): void
 
         $tz = TimeZoneUtil::getTimeZone('foo', Reader::read($vobj));
         $ex = new \DateTimeZone(date_default_timezone_get());
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 
     public function testLjubljanaBug(): void
@@ -319,7 +319,7 @@ public function testLjubljanaBug(): void
 
         $tz = TimeZoneUtil::getTimeZone('/freeassociation.sourceforge.net/Tzfile/Europe/Ljubljana', Reader::read($vobj));
         $ex = new \DateTimeZone('Europe/Ljubljana');
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 
     public function testWeirdSystemVLICs(): void
@@ -367,13 +367,13 @@ public function testWeirdSystemVLICs(): void
 
         $tz = TimeZoneUtil::getTimeZone('/freeassociation.sourceforge.net/Tzfile/SystemV/EST5EDT', Reader::read($vobj), true);
         $ex = new \DateTimeZone('America/New_York');
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 
     public function testPrefixedOffsetExchangeIdentifier(): void
     {
         $tz = TimeZoneUtil::getTimeZone('(UTC-05:00) Eastern Time (US & Canada)');
         $ex = new \DateTimeZone('America/New_York');
-        $this->assertEquals($ex->getName(), $tz->getName());
+        self::assertEquals($ex->getName(), $tz->getName());
     }
 }
diff --git a/tests/VObject/UUIDUtilTest.php b/tests/VObject/UUIDUtilTest.php
index e792c454..170107b3 100644
--- a/tests/VObject/UUIDUtilTest.php
+++ b/tests/VObject/UUIDUtilTest.php
@@ -8,16 +8,16 @@ class UUIDUtilTest extends TestCase
 {
     public function testValidateUUID(): void
     {
-        $this->assertTrue(
+        self::assertTrue(
             UUIDUtil::validateUUID('11111111-2222-3333-4444-555555555555')
         );
-        $this->assertFalse(
+        self::assertFalse(
             UUIDUtil::validateUUID(' 11111111-2222-3333-4444-555555555555')
         );
-        $this->assertTrue(
+        self::assertTrue(
             UUIDUtil::validateUUID('ffffffff-2222-3333-4444-555555555555')
         );
-        $this->assertFalse(
+        self::assertFalse(
             UUIDUtil::validateUUID('fffffffg-2222-3333-4444-555555555555')
         );
     }
@@ -27,7 +27,7 @@ public function testValidateUUID(): void
      */
     public function testGetUUID(): void
     {
-        $this->assertTrue(
+        self::assertTrue(
             UUIDUtil::validateUUID(
                 UUIDUtil::getUUID()
             )
diff --git a/tests/VObject/VCard21Test.php b/tests/VObject/VCard21Test.php
index 16ee46a6..d091e35d 100644
--- a/tests/VObject/VCard21Test.php
+++ b/tests/VObject/VCard21Test.php
@@ -22,7 +22,7 @@ public function testPropertyWithNoName(): void
         $vobj = Reader::read($input);
         $output = $vobj->serialize();
 
-        $this->assertEquals($input, $output);
+        self::assertEquals($input, $output);
     }
 
     public function testPropertyPadValueCount(): void
@@ -46,6 +46,6 @@ public function testPropertyPadValueCount(): void
 
 VCF;
 
-        $this->assertEquals($expected, $output);
+        self::assertEquals($expected, $output);
     }
 }
diff --git a/tests/VObject/VCardConverterTest.php b/tests/VObject/VCardConverterTest.php
index 7af60b7b..9db0ad56 100644
--- a/tests/VObject/VCardConverterTest.php
+++ b/tests/VObject/VCardConverterTest.php
@@ -49,7 +49,7 @@ public function testConvert30to40(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD40);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -90,7 +90,7 @@ public function testConvert40to40(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD40);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -132,7 +132,7 @@ public function testConvert21to40(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD40);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -175,7 +175,7 @@ public function testConvert30to30(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD30);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -219,7 +219,7 @@ public function testConvert40to30(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD30);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -255,7 +255,7 @@ public function testConvertGroupCard(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD40);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -275,7 +275,7 @@ public function testConvertGroupCard(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD30);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -307,7 +307,7 @@ public function testBDAYConversion()
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD40);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -325,7 +325,7 @@ public function testBDAYConversion()
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD30);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -403,7 +403,7 @@ public function testConvertIndividualCard(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD30);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -420,7 +420,7 @@ public function testConvertIndividualCard(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD40);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -453,7 +453,7 @@ public function testAnniversary(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD30);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -471,7 +471,7 @@ public function testAnniversary(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD40);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -512,7 +512,7 @@ public function testMultipleAnniversaries(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD30);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -530,7 +530,7 @@ public function testMultipleAnniversaries(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD40);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
@@ -554,7 +554,7 @@ public function testNoLabel(): void
 
         $vcard = Reader::read($input);
 
-        $this->assertInstanceOf(Component\VCard::class, $vcard);
+        self::assertInstanceOf(Component\VCard::class, $vcard);
         /** @var VCard $vcard */
         $vcard = $vcard->convert(Document::VCARD40);
         $vcard = $vcard->serialize();
@@ -577,7 +577,7 @@ public function testNoLabel(): void
 
 VCF;
 
-        $this->assertEquals($expected, str_replace("\r", '', $vcard));
+        self::assertEquals($expected, str_replace("\r", '', $vcard));
     }
 
     /**
@@ -608,7 +608,7 @@ public function testPhoneNumberValueTypeGetsRemoved(): void
         $vcard = Reader::read($input);
         $vcard = $vcard->convert(Document::VCARD40);
 
-        $this->assertVObjectEqualsVObject(
+        self::assertVObjectEqualsVObject(
             $output,
             $vcard
         );
diff --git a/tests/VObject/VersionTest.php b/tests/VObject/VersionTest.php
index 9e18b45e..f2f240eb 100644
--- a/tests/VObject/VersionTest.php
+++ b/tests/VObject/VersionTest.php
@@ -9,6 +9,6 @@ class VersionTest extends TestCase
     public function testString(): void
     {
         $v = Version::VERSION;
-        $this->assertEquals(-1, version_compare('2.0.0', $v));
+        self::assertEquals(-1, version_compare('2.0.0', $v));
     }
 }
diff --git a/tests/VObject/WriterTest.php b/tests/VObject/WriterTest.php
index 42cde6a9..f5cd7a69 100644
--- a/tests/VObject/WriterTest.php
+++ b/tests/VObject/WriterTest.php
@@ -16,19 +16,19 @@ public function getComponent(): ?Document
     public function testWriteToMimeDir(): void
     {
         $result = Writer::write($this->getComponent());
-        $this->assertEquals("BEGIN:VCALENDAR\r\nEND:VCALENDAR\r\n", $result);
+        self::assertEquals("BEGIN:VCALENDAR\r\nEND:VCALENDAR\r\n", $result);
     }
 
     public function testWriteToJson(): void
     {
         $result = Writer::writeJson($this->getComponent());
-        $this->assertEquals('["vcalendar",[],[]]', $result);
+        self::assertEquals('["vcalendar",[],[]]', $result);
     }
 
     public function testWriteToXml(): void
     {
         $result = Writer::writeXml($this->getComponent());
-        $this->assertEquals(
+        self::assertEquals(
             '<?xml version="1.0" encoding="UTF-8"?>'."\n".
             '<icalendar xmlns="urn:ietf:params:xml:ns:icalendar-2.0">'."\n".
             ' <vcalendar/>'."\n".